Otherwise yields 0. pmap pmap(F, S) is like map(F, S) except that (if multithreading is in force) each element of the result is evaluated in parallel prefix prefix(N, L) yields the first N items in L. If L does not have N items, yields all of L. [incremental] reduce reduce(B, U, S) Here B is a binary operator with unit U and S is an array or list.