FOM: straification

[Thomas Forster]

A minor detail: i gathered from Marco Forti that the idea of a model
of set theory as a set of atoms with an injection into its power set
is due to his mentor di Giorgi.  That way one can motivate stratification 
very easily: you nail down the truth-values of the stratified formulae
once you have determined even the *range* of the injection: to nail down
the truth-values of the *un*stratified expressions you need to get your 
hands on the *graph* of the function as well.

        Thomas Forster