`Multiply` returns as value a sequence of integers created by
multiplying the `First` element of the path with each element in (the
integer prefix of) the `Rest`. `Multiply` fails if the path prefix
contains less than two integers.

Multiply:<2 3 5 7> = 6 10 14. Multiply:<-2 3 -5 foo 7> = -6 10 foo 7.