When applied to an equational theorem, LEFT_LIST_PBETA applies paired
beta-reduction over a top-level chain of beta-redexes to the left-hand side
(only).
Variables are renamed if necessary to avoid free variable capture.
A |- (\p1...pn. t) q1 ... qn = s
---------------------------------- LEFT_LIST_BETA
A |- t[q1/p1]...[qn/pn] = s