Accumulators (review)
lCertain arguments of functions, particularly tail-recursive ones, are often designated as ÒaccumulatorsÓ, e.g.

lThe idea is that this argument value ÒaccumulatesÓ until the function is ready to return the answer without recursing.
f1(a, n, x) = x <= n ?         f1(a*x, n, x+1)
    : a;
accumulator argument