Restore sf predicates, not scode predicates.

Restore sf predicates, not scode predicates.

Various parts of sf were silently disabled in the overly aggressive
change

   commit 816b9122751c9b60b85f9ce3db0df8a516f763cb
   Author: Chris Hanson <org/chris-hanson/cph>
   Date:   Wed Jan 24 00:07:59 2018 -0800

       Greatly simplify SCode abstraction and change names to contain "scode".

       Also remove all FOO-components and FOO-subexpressions procedures.
       More work remains: the lambda abstraction is an unholy mess and needs to be
       cleaned up.  The scan-defines stuff also merits some consideration.

which kinda reflects on the sensibility of using disjoint type
predicates across domains like this as one does in Scheme.