arrow notation fixes (problems reported bu John Hughes):

* allow an infixexp (exp0) to the left of -<.  This adds 4 more
  shift/reduce conflicts, because it makes if/lambda/let/case/proc
  before -< ambiguous.  This is the same sort of thing as
  "if x then y else z + 1", and as there shifting does the right thing.

* described the grammar more precisely

* fixed an example

merge to STABLE