experimental GP feature: = in comprehension

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

Dear GP developers,

I have added a new experimental GP feature which allow to
use = in comprehension to introduce new variables.

For example:

? [[b,a]|b<-[1..10];a<-[b!+1],issquare(b)]
%3 = [[1,2],[4,25],[9,362881]]

can be written as

? [[b,a]|b<-[1..10];a=b!+1,issquare(b)]
%4 = [[1,2],[4,25],[9,362881]]

Cheers,
Bill