--------------------------
	Correct a bug in exprArity
	--------------------------

This long-standing bug meant that exprArity gave over-pessimistic
answers.  e.g.

	\x -> f x

where nothing is known about f.  We were getting

	0 (for f)
	-1 (for the argument)
	+1 (for the lambda)
	-----
	0

The right answer is of course 1.