input. If the input is simple (the first three `match` cases), we
return it without further processing. But if the input is complex, we
first process the subexpressions, and only then see if we have a redex
input. If the input is simple (the first three `match` cases), we
return it without further processing. But if the input is complex, we
first process the subexpressions, and only then see if we have a redex