fib

import std.array: array;
auto fibs = iota(1, 21).map!(n => fib(n)).array;
assert(fibs == [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144,
                233, 377, 610, 987, 1597, 2584, 4181, 6765]);