List reversal
= List reversal
Reverse the order of a finite <list>. Reversal is an <involution> and satisfies $(AB)^R=B^RA^R$ for <list concatenation>. In an <array> segment of length $n$, exchanging symmetric offsets $j$ and $n-1-j$ for $0\leq j<\lfloor n/2\rfloor$ realizes reversal in constant auxiliary space.