doc-src/Tutorial/Recdef/last

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

consts last :: 'a list => 'a
recdef last "measure (%xs. length xs)"
  "last [x]      = x"
  "last (x#y#zs) = last (y#zs)"