pub fn zeta_expand_head_term(t: &Term) -> Option<Term>Expand description
ζ-expand a Let at the head, whatever its occurrence count — the step a conversion check
must be able to take on demand once zeta_or_share lets a Let survive into a normal
form. None for anything that is not Let-headed.
The Term-side twin of TermStore::zeta_expand_head.