ambages.selectors

Selectors and constructor functions (which are only there for readability)
built from CARs, CDRs and CONSes.

bind

(bind x y e)

bond

(bond x e)

but-first-goal

(but-first-goal x)

lvl

(lvl x)
selector: extract level from molec

molec

(molec x y)
constructor: a `molec` is a tuple of (level,prolog_exp), where level
is used to discriminate variables at different levels in the proof tree.

xpr

(xpr x)
selector: extract prolog expression