Read the nth element of the truth4arr array.
In the execution this is an array access, but the logical
definition is just a thin wrapper for nth:
truth4s$ai (stobjs::i truth4arr$a)
(declare (xargs :guard (and (truth4arr$ap truth4arr$a)
(<= 0 stobjs::i)
(ec-call (truth4-fix (ec-call (nth stobjs::i truth4arr$a)))))