Fix (and test) for SourceForge bug 1161624: erroneous rejection of

	foo = proc x -> arr (\y -> y-1) -< x

Now open a new level for the left side of -<, so that variables bound
in the proc are illegal, but variables bound in the expression are OK.

Note that the levels gimmick doesn't really implement holes in the
scope: it rules out nasty obfuscations like

	foo x = proc x -> arr (\y -> x-1) -< x

Also added the same treatment to the head of a `form', where it was
missing.

(for STABLE)