doc-src/Tutorial/CodeGen/expr

Removed ~10000 hack in function idx that can lead to inconsistencies
when unifying terms with a large number of abstractions.

types 'v binop = 'v => 'v => 'v
datatype ('a,'v) expr = Cex 'v
                      | Vex 'a
                      | Bex ('v binop)  (('a,'v) expr) (('a,'v) expr)