The function r : L → L, where L denotes strings of characters from some alphabet, defined in such a way that r reverses the order of the elements in its parameter. If & denotes concatenation of strings, then
if
s is null or a single character and
The idea can be extended to include reversing of items in a list, of items in some sequence, or of items in an arbitrary one-dimensional array. Reversal is an involution operation.