lskept: the old selection check required at least one non-variable
non-closed type in the lsymbol's signature. This is too permissive,
because (list_match : list 'a -> 'b -> 'b) is allowed and will produce
an instance for every type in the "inst" set. The new check requires
that all freely standing type variables occur under a type constructor
elsewhere in the lsymbol's signature.

lsinst: accept any monomorphic instance of any polymorphic symbol
except equality. The old procedure applied the same restrictions
as for the "lskept" set which serve no real purpose for "lsinst".