Compilers

Start Lecture #2

Given a grammar, parsing a string (of terminals) consists of determining if the string is in the language generated by the grammar. If it is in the language, parsing produces a derivation. If it is not, parsing reports an error.

The opposite of derivation is reduction. Given a production, the LHS produces or derives the RHS (a derivation) and the RHS is reduced to the LHS (a reduction).

Ignoring errors for the moment, parsing a string means reducing the string to the start symbol or equivalently deriving the string from the start symbol.

Homework: 1a, 1c, 2a-c (don't worry about justifying your answers).

Remark: Since we are in section 2.2 these questions are in section 2.2.7, the last subsection of section 2.2.

2.2.3: Parse trees

While deriving 7+4-5, one could produce the Parse Tree shown on the right.

You can read off the productions from the tree. For any internal (i.e., non-leaf) tree node, its children give the right hand side (RHS) of a production having the node itself as the LHS.

The leaves of the tree, read from left to right, is called the yield of the tree. We say that this string is derived from the (nonterminal at the) root, or is generated by the root, or can be reduced to the root. The tree on the right shows that 7+4-5 can be derived from list.

Homework: 1b

2.2.4: Ambiguity

An ambiguous grammar is one in which there are two or more parse trees yielding the same final string. We wish to avoid such grammars.

The grammar above is not ambiguous. For example 1+2+3 can be parsed only one way; the arithmetic must be done left to right. Note that I am not giving a rule of arithmetic, just of this grammar. If you reduced 2+3 to list you would be stuck since it is impossible to further reduce 1+list (said another way it is not possible to derive 1+list from the start symbol).

Remark:
The following is a wrong proof of ambiguity. Consider the grammar
S → A B
A → x
B → x
This grammar is ambiguous because we can derive the string x x in two ways
S → A B → A x → x x
S → A B → x B → x x
WRONG!!
There are indeed two derivations, but they have the same parse tree!
End of Remark.

Homework: 3 (applied only to parts a, b, and c of 2)

2.2.5: Associativity of operators

Our grammar gives left associativity. That is, if you traverse the parse tree in postorder and perform the indicated arithmetic you will evaluate the string left to right. Thus 8-8-8 would evaluate to -8. If you wished to generate right associativity (normally exponentiation is right associative, so 2**3**2 gives 512 not 64), you would change the first two productions to

  list → digit + list
  list → digit - list

Draw in class the parse tree for 7+4-5 with this new grammar.

2.2.6: Precedence of operators

We normally want * to have higher precedence than +. We do this by using an additional nonterminal to indicate the items that have been multiplied. The example below gives the four basic arithmetic operations their normal precedence unless overridden by parentheses. Redundant parentheses are permitted. Equal precedence operations are performed left to right.

  expr   → expr + term | expr - term | term
  term   → term * factor | term / factor | factor
  factor → digit | ( expr )
  digit  → 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

Do the examples 1+2/3-4*5 and (1+2)/3-4*5 on the board.

Note how the precedence is enforced by the grammar; slick!

Statements

Keywords are very helpful for distinguishing statements from one another.

    stmt → id := expr
    	 | if expr then stmt
    	 | if expr then stmt else stmt
    	 | while expr do stmt
    	 | begin opt-stmts end
    opt-stmts → stmt-list | ε
    stmt-list → stmt-list ; stmt | stmt
  

Remarks:

1. In the above example I underlined the nonterminals. This is not normally done. It is easy to tell the nonterminals; they are the symbols that appear on the LHS.
2. opt-stmts stands for optional statements. The begin-end block can be empty in some languages.
3. The ε (epsilon) stands for the empty string.
4. The use of epsilon productions will add complications.
5. Some languages do not permit empty blocks. For example, Ada has a null statement, which does nothing when executed, but avoids the need for empty blocks.
6. The above grammar is ambiguous!
7. The notorious dangling else problem.
8. How do you parse if x then if y then z=1 else z=2?

Homework: 4 a-d (for a the operands are digits and the operators are +, -, *, and /).

2.3: Syntax-Directed Translation

The idea is to specify the translation of a source language construct in terms of attributes of its syntactic components. The basic idea is use the productions to specify a (typically recursive) procedure for translation. For example, consider the production

    stmt-list → stmt-list ; stmt
  

1. Call ourselves recursively to process the right stmt-list (which is smaller). This will, say, generate code for all the statements in the right stmt-list.
2. Call the procedure for stmt, generating code for stmt.
3. Process the left stmt-list by combining the results for the first two steps as well as what is needed for the semicolon (a terminal, so we do not further delegate its actions). In this case we probably concatenate the code for the right stmt-list and stmt.

To avoid having to say the right stmt-list and the left stmt-list we write the production as

    stmt-list → stmt-list1 ; stmt
  

Question: Why won't this go on forever?
Answer: Eventually stmt-list₁ will consist of only one stmt and then one of the other production for stmt-list will be used.

2.3.1: Postfix Notation (An Example)

This notation is called postfix because the rule is operator after operand(s). Parentheses are not needed. The notation we normally use is called infix because the rules is operator in between operands. If you start with an infix expression, the following algorithm will give you the equivalent postfix expression.

• Variables and constants are left alone.
• E op F becomes E' F' op, where E' and F' are the postfix of E and F respectively.
• ( E ) becomes E', where E' is the postfix of E.

One question is, given say 1+2-3, what are E, F and op? Does E=1+2, F=3, and op=-? Or does E=1, F=2-3 and op=+? This is the issue of precedence and associativity mentioned above. To simplify the present discussion we will start with fully parenthesized infix expressions.

Example: 1+2/3-4*5

1. Start with 1+2/3-4*5
2. Parenthesize (using standard precedence) to get (1+(2/3))-(4*5)
3. Apply the above rules to calculate P{(1+(2/3))-(4*5)}, where P{X} means convert the infix expression X to postfix.
   1. P{(1+(2/3))-(4*5)}
   2. P{(1+(2/3))} P{(4*5)} -
   3. P{1+(2/3)} P{4*5} -
   4. P{1} P{2/3} + P{4} P{5} * -
   5. 1 P{2} P{3} / + 4 5 * -
   6. 1 2 3 / + 4 5 * -

Example: Now do (1+2)/3-4*5

1. Parenthesize to get ((1+2)/3)-(4*5)
2. Calculate P{((1+2)/3)-(4*5)}
   1. P{((1+2)/3) P{(4*5)} -
   2. P{(1+2)/3} P{4*5) -
   3. P{(1+2)} P{3} / P{4} P{5} * -
   4. P{1+2} 3 / 4 5 * -
   5. P{1} P{2} + 3 / 4 5 * -
   6. 1 2 + 3 / 4 5 * -

2.3.2: Synthesized Attributes

We want to decorate the parse trees we construct with annotations that give the value of certain attributes of the corresponding node of the tree.

Later in the semester, we will use as input an algol/ada/C-like language and will have a code attribute for many nodes with the property that code contains the intermediate code that results from compiling the program corresponding to the leaves of the subtree rooted at this node. In particular,

However, it is now only the beginning of the semester so our goal is more modest. We will do the example of translating infix to postfix using the same infix grammar as above. For convenience, the grammar is repeated just below. The names of the nonterminals correspond to standard arithmetic terminology where one multiplies and divides factors to obtain terms, which in turn are added and subtracted to form expressions.

  expr   → expr + term | expr - term | term
  term   → term * factor | term / factor | factor
  factor → digit | ( expr )
  digit  → 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

This grammar supports parentheses, although our example 1+2/3-4*5 does not use them. On the right is a movie in which the parse tree is built from this example.

Question: Was this a top-down or bottom-up movie?

The attribute we will associate with the nodes is the postfix form of the string in the leaves below the node. In particular, the value of this attribute at the root is the postfix form of the entire source.

The book does a simpler grammar (no *, /, or parentheses) for a simpler example. You might find that one easier.

Syntax-Directed Definitions (SDDs)

Definition: A syntax-directed definition is a grammar together with semantic rules associated with the productions. These rules are used to compute attribute values. A parse tree augmented with the attribute values at each node is called an annotated parse tree.

For the bottom-up approach I will illustrate now, we annotate a node after having annotated its children. Thus the attribute values at a node can depend on the values of attributes at the children of the node but not on attributes at the parent of the node. We call such bottom-up attributes synthesized, since they are formed by synthesizing the attributes of the children.

In chapter 5, when we study top-down annotations as well, we will introduce inherited attributes that are passed down from parents to children.

We specify how to synthesize attributes by giving the semantic rules together with the grammar. That is, we give the syntax directed definition.

We apply these rules bottom-up (starting with the geographically lowest productions, i.e., the lowest lines in the tree) and get the annotated graph shown on the right. The annotation are drawn in green.

Homework: Draw the annotated graph for (1+2)/3-4*5.

2.3.3: Simple Syntax-Directed Definitions

If the semantic rules of a syntax-directed definition all have the property that the new annotation for the left hand side (LHS) of the production is just the concatenation of the annotations for the nonterminals on the RHS in the same order as the nonterminals appear in the production, we call the syntax-directed definition simple. It is still called simple if new strings are interleaved with the original annotations. So the example just done is a simple syntax-directed definition.

Remark: SDD's feature semantic rules. We will soon learn about Translation Schemes, which feature a related concept called semantic actions. When one has a simple SDD, the corresponding translation scheme can be done without constructing the parse tree. That is, while doing the parse, when you get to the point where you would construct the node, you just do the actions. In the translation scheme corresponding to the present example, the action at a node is just to print the new strings at the appropriate points.

2.3.4: (depth-first) Tree Traversals

When performing a depth-first tree traversal, it is clear in what order the leaves are to be visited, namely left to right. In contrast there are several choices as to when to visit an interior (i.e. non-leaf) node. The traversal can visit an interior node

1. Before visiting any of its children.
2. Between visiting its children.
3. After visiting all of its children.

I do not like the book's pseudocode as I feel the names chosen confuse the traversal with visiting the nodes. I prefer the pseudocode below, which uses the following conventions.

• Comments are introduced by -- and terminate at the end of the line (as in the programming language Ada).
  
  
• Indenting is significant so begin/end or {} are not used (from the programming language family B2/ABC/Python)

    traverse (n : treeNode)
        if leaf(n)                      -- visit leaves once; base of recursion
           visit(n)
        else                            -- interior node, at least 1 child
           -- visit(n)                  -- visit node PRE visiting any children
           traverse(first child)        -- recursive call
           while (more children remain) -- excluding first child
               -- visit(n)              -- visit node IN-between visiting children
               traverse (next child)    -- recursive call
           -- visit(n)                  -- visit node POST visiting all children
  

Note the following properties

1. As written, with the last three visit()s commented out, only the leaves are visited and those visits are in left to right order.
   
   
2. If you uncomment just the first (interior node) visit, you get a preorder traversal, in which each node is visited before (i.e., pre) visiting any of its children.
   
   
3. If you uncomment just the last visit, you get a postorder traversal, in which each node is visited after (i.e., post) visiting all of its children.
   
   
4. If you uncomment only the middle visit, you get an inorder traversal, in which the node is visited (in-) between visiting its children.
   
   Inorder traversals are normally defined only for binary trees, i.e., trees in which every interior node has exactly two children. Although the code with only the middle visit uncommented works for any tree, we will, like everyone else, reserve the name inorder traversal for binary trees. In the case of binary search trees (everything in the left subtree is smaller than the root of that subtree, which in tern is smaller than everything in the corresponding right subtree) an inorder traversal visits the values of the nodes in (numerical) order.
   
   
5. If you uncomment two of the three visits, you get a traversal without a name.
   
   
6. If you uncomment all of the three visits, you get an Euler-tour traversal.

To explain the name Euler-tour traversal, recall that an Eulerian tour on a directed graph is one that traverses each edge once. If we view the tree on the right as undirected and replace each edge with two arcs, one in each direction, we see that the pink curve is indeed an Eulerian tour. It is easy to see that the curve visits the nodes in the order of the pseudocode (with all visits uncommented).

Normally, the Euler-tour traversal is defined only for a binary tree, but this time I will differ from convention and use the pseudocode above to define Euler-tour traversal for all trees.

Note the following points about our Euler-tour traversal.

• A node with k children is visited k+1 times. The diagram shows nodes with 0, 1, 2, and 3 children.
• In a binary tree, a leaf is visited once and an interior node is visited three times. This is one of the standard definitions of an Euler-tour traversal for a binary tree.
• The other standard definition has all nodes visited 3 times. For a leaf the three visits are in succession. Modifying the pseudocode to obtain this definition simply requires replacing the leaf visit with

  	visit(n); visit(n); visit(n)
        

Remarks

• Since, at this point in the course, we are considering only synthesized attributes, a postorder traversal will always yield a correct evaluation order for the attributes. This is so since synthesized attributes depend only on attributes of child nodes and a postorder traversal visits a node only after all the children have been visited (and hence all the child node attributes have been evaluated).
• In the general case (when not all attributes are synthesized), SDDs do not specify an evaluation order for the attributes of the parse tree. The requirement remains that each attribute is evaluated after all those that it depends on. This general case is quite difficult, and sometimes no such order is possible.

2.3.5: Translation schemes

The bottom-up annotation scheme just described generates the final result as the annotation of the root. In our infix to postfix example we get the result desired by printing the root annotation. Now we consider another technique that produces its results incrementally.

Instead of giving semantic rules for each production (and thereby generating annotations) we can embed program fragments called semantic actions within the productions themselves.

When drawn in diagrams (e.g., see the diagram below), the semantic action is connected to its node with a distinctive, often dotted, line. The placement of the actions determine the order they are performed. Specifically, one executes the actions in the order they are encountered in a depth-first traversal of the tree (the children of a node are visited in left to right order). Note that these action nodes are all leaves and hence they are encountered in the same order for both preorder and postorder traversals (and inorder and Euler-tree order).

Definition: A syntax-directed translation scheme is a context-free grammar with embedded semantic actions.

In the SDD for our infix to postfix translator, the parent either

1. takes the attribute of its only child or
2. concatenates the attributes left to right of its several children and adds something at the end.

Emitting a Translation

The table on the right gives the semantic actions corresponding to a few of the rows of the table above. Note that the actions are enclosed in {}. The corresponding semantic rules are given as well.

It is redundant to give both semantic actions and semantic rules; in practice, we use one or the other. In this course we will emphasize semantic rules, i.e. syntax directed definitions (SDDs). I show both the rules and the actions in a few tables just so that we can see the correspondence.

The diagram for 1+2/3-4*5 with attached semantic actions is shown on the right.

Given an input, e.g. our favorite 1+2/3-4*5, we just do a (left-to-right) depth first traversal of the corresponding diagram and perform the semantic actions as they occur. When these actions are print statements as above, we are said to be emitting the translation.

Since the actions are all leaves of the tree, they occur in the same order for any depth-first (left-to-right) traversal (e.g., postorder, preorder, or Euler-tour order).

Do on the board a depth first traversal of the diagram, performing the semantic actions as they occur, and confirm that the translation emitted is in fact 123/+45*-, the postfix version of 1+2/3-4*5

Homework: Produce the corresponding diagram for (1+2)/3-4*5.

Prefix to infix translation

When we produced postfix, all the prints came at the end (so that the children were already printed). The { action }'s do not need to come at the end. We illustrate this by producing infix arithmetic (ordinary) notation from a prefix source.

In prefix notation the operator comes first. For example, +1-23 evaluates to zero and +-123 evaluates to 2. Consider the following grammar, which generates the simple language of prefix expressions consisting of addition and subtraction of digits between 1 and 3 without parentheses (prefix notation and postfix notation do not use parentheses).

    P → + P P | - P P | 1 | 2 | 3
  

The resulting parse tree for +1-23 with the semantic actions attached is shown on the right. Note that the output language (infix notation) has parentheses.

The table below shows both the semantic actions and rules used by the translator. As mentioned previously, one normally does not use both actions and rules.

Prefix to infix translator

Production with Semantic Action	Semantic Rule
P → + { print('(') } P1 { print(')+(') } P2 { print(')') }	P.t := '(' || P1.t || ')+(' || P.t || ')'
P → - { print('(') } P1 { print(')-(') } P2 { print(')') }	P.t := '(' || P1.t || ')-(' || P.t || ')'
P → 1 { print('1') }	P.t := '1'
P → 2 { print('2') }	P.t := '2'
P → 3 { print('3') }	P.t := '3'

First do a preorder traversal of the tree and see that you get 1+(2-3). In fact you don't get that answer, but instead get a fully parenthesized version that is equivalent.

Next start a postorder traversal and see that it produces the same output (i.e., executes the same prints in the same order).

Question: What about an Euler-tour order?

Answer: The same result. For all traversals, the leaves are printed in left to right order and all the semantic actions are leaves.

Finally, pretend the prints aren't there, i.e., consider the unannotated parse tree and perform a postorder traversal, evaluating the semantic rules at each node encountered. Postorder is needed (and sufficient) since we have synthesized attributes and hence having child attributes evaluated prior to evaluating parent attributes is both necessary and sufficient to ensure that whenever an attribute is evaluated all the component attributes have already been evaluated. (It will not be so easy in chapter 5, when we have inherited attributes as well.)

Homework: 2.

2.4: Parsing

Objective: Given a string of tokens and a grammar, produce a parse tree yielding that string (or at least determine if such a tree exists).

We will learn both top-down (begin with the start symbol, i.e. the root of the tree) and bottom up (begin with the leaves) techniques.

In the remainder of this chapter we just do top down, which is easier to implement by hand, but is less general. Chapter 4 covers both approaches.

Tools (so called parser generators) often use bottom-up techniques.

In this section we assume that the lexical analyzer has already scanned the source input and converted it into a sequence of tokens.

2.4.1: Top-down parsing

Consider the following simple language, which derives a subset of the types found in the (now somewhat dated) programming language Pascal. I do not assume you know pascal.

We have two nonterminals, type, which is the start symbol, and simple, which represents the simple types.

There are 8 terminals, which are tokens produced by the lexer and correspond closely with constructs in pascal itself. Specifically, we have.

1. integer and char
2. id for identifier
3. array and of used in array declarations
4. ↑ meaning pointer to
5. num for a (positive whole) number
6. dotdot for .. (used to give a range like 6..9)

The productions are

    type   → simple
    type   → ↑ id
    type   → array [ simple ] of type
    simple → integer
    simple → char
    simple → num dotdot num
  

Parsing is easy in principle and for certain grammars (e.g., the one above) it actually is easy. We start at the root since this is top-down parsing and apply the two fundamental steps.

1. At the current (nonterminal) node, select a production whose LHS is this nonterminal and whose RHS matches the input at this point. Make the RHS the children of this node (one child per RHS symbol).
2. Go to the next node needing a subtree.

When programmed this becomes a procedure for each nonterminal that chooses a production for the node and calls procedures for each nonterminal in the RHS of that production. Thus it is recursive in nature and descends the parse tree. We call these parsers recursive descent.

The big problem is what to do if the current node is the LHS of more than one production. The small problem is what do we mean by the next node needing a subtree.

The movie on the right, which succeeds in parsing, works by tossing 2 ounces of pixie dust into the air and choosing the production onto which the most dust falls. (An alternative interpretation is given below.)

The easiest solution to the big problem would be to assume that there is only one production having a given nonterminal as LHS. There are two possibilities

1. No circularity. For example

   	expr → term + term
   	term → factor / factor
   	factor → digit
   	digit → 7
         

   But this is very boring. The only possible sentence is 7/7+7/7
   
   
2. Circularity

   	expr → term + term
   	term → factor / factor
   	factor → ( expr )
         

   This is even worse; there are no (finite) sentences. Only an infinite sentence beginning (((((((((.

So this won't work. We need to have multiple productions with the same LHS.

How about trying them all? We could do this! If we get stuck where the current tree cannot match the input we are trying to parse, we would backtrack.

Instead, we will look ahead one token in the input and only choose productions that can yield a result starting with this token. Furthermore, we will (in this section) restrict ourselves to predictive parsing in which there is only one production that can yield a result starting with a given token. This solution to the big problem also solves the small problem. Since we are trying to match the next token in the input, we must choose the leftmost (nonterminal) node to give children to.

2.4.2: Predictive parsing

Let's return to pascal array type grammar and consider the three productions having type as LHS. Remember that, even when I write the short form

    type → simple | ↑ id | array [ simple ] of type
  

For each production P we wish to construct the set FIRST(P) consisting of those tokens (i.e., terminals) that can appear as the first symbol of a string derived from the RHS of P.

FIRST is actually defined on strings not productions. When I write FIRST(P), I really mean FIRST(RHS). Similarly, I often say the first set of the production P when I should really say the first set of the RHS of the production P. Formally, we proceed as follows.

Let α be a string of terminals and/or nonterminals. FIRST(α) is the set of terminals that can appear as the first symbol in a string of terminals derived from α. If α is ε or α can derive ε, then ε is in FIRST(α)

So given α we find all strings of terminals that can be derived from α and pick off the first terminal from each string (as often happens, ε requires a special case).

Question: How do we calculate FIRST(α)?

Answer: Wait until chapter 4 for a formal algorithm. For these simple examples it is reasonably clear.

Definition: Let r be the RHS of a production P. FIRST(P) is FIRST(r).

To use predictive parsing, we make the following

Assumption: Let P and Q be two productions with the same LHS, Then FIRST(P) and FIRST(Q) are disjoint. Thus, if we know both the LHS and the token that must be first, there is (at most) one production we can apply. BINGO!

An example of predictive parsing

This table gives the FIRST sets for our pascal array type example.

Production	FIRST
type → simple	{ integer, char, num }
type → ↑ id	{ ↑ }
type → array [ simple ] of type	{ array }
simple → integer	{ integer }
simple → char	{ char }
simple → num dotdot num	{ num }

Make sure that you understand how this table was derived. It is not yet clear how to calculate FIRST for a complicated example. We will learn the general procedure in chapter 4.

Note that the three productions with type as LHS have disjoint FIRST sets. Similarly the three productions with simple as LHS have disjoint FIRST sets. Thus predictive parsing can be used. We process the input left to right and call the current token lookahead since it is how far we are looking ahead in the input to determine the production to use. The movie on the right shows the process in action.

Homework:

A. Construct the FIRST sets for

    rest → + term rest | - term rest | term
    term → 1 | 2 | 3
  

End of Homework.

2.4.3: When to Use ε-productions

Not all grammars are as friendly as the last example. The first complication is when ε occurs as a RHS. If this happens or if the RHS can generate ε, then ε is included in FIRST.

But ε would always match the current input position!

The rule is that if lookahead is not in FIRST of any production with the desired LHS, we use the (unique!) production (with that LHS) that has ε in FIRST.

The text does a C instead of a pascal example. The productions are

    stmt → expr ;
    	 | if ( expr ) stmt
    	 | for ( optexpr ; optexpr ; optexpr ) stmt
    	 | other
    optexpr → expr | ε
  

For completeness, on the right is the beginning of a movie for the C example. Note the use of the ε-production at the end since no other entry in FIRST will match ;

Once again, the full story will be revealed in chapter 4 when we do parsing in a more complete manner.

2.4.4: Designing a Predictive Parser

Predictive parsers are fairly easy to construct as we will now see. Since they are recursive descent parsers we go top-down with one procedure for each nonterminal. Do remember that to use predictive parsing, we must have disjoint FIRST sets for all the productions having a given nonterminal as LHS.

1. For each nonterminal, write a procedure that chooses the unique(!) production having lookahead in its FIRST set. Use the ε production if no other production matches. If no production matches and there is no ε production, the parse fails.
2. Having chosen a production, these procedures then mimic the RHS of the production. They call procedures for each nonterminal and call match for each terminal.
3. Write a procedure match(terminal) that advances lookahead to the next input token after confirming that the previous value of lookahead equals the terminal argument.
4. Write a main program that initializes lookahead to the first input token and invokes the procedure for the start symbol.

The book has code at this point, which you should read.

2.4.5: Left Recursion

Another complication. Consider

    expr → expr + term
    expr → term
  

For the first production the RHS begins with the LHS. This is called left recursion. If a recursive descent parser would pick this production, the result would be that the next node to consider is again expr and the lookahead has not changed. An infinite loop occurs. (Also note that the first sets are not disjoint.)

Note that this is NOT a problem with the grammar per se, but is a limitation of predictive parsing. For example if we had the additional production

    term → x
  

    x + x
  

If the grammar were instead

    expr → term + expr
    expr → term
  

Consider, instead of the original (left-recursive) grammar, the following replacement

    expr → term rest
    rest → + term rest
    rest → ε
  

Both sets of productions generate the same possible token strings, namely

    term + term + ... + term
  

Using this technique to eliminate left-recursion will (next month) make it harder for us to get left associativity for arithmetic, but we shall succeed!

In general, for any nonterminal A, and any strings α, and β (α and β cannot start with A), we can replace the pair of productions

    A → A α | β
  

    A → β R
    R → α R | ε
  

For the example above A is expr, R is rest, α is + term, and β is term.

Yes, this looks like magic.
Yes, there are more general possibilities.
We will have more to say in chapter 4.