Recamán's sequence

n = 0
an =
{
a0 = initial value
back = if back > 0 & unused
forward = otherwise
terms = n = 1 … N
use: a[n-1], n, Math.sin, Math.floor, Math.abs, …
examples: a[n-1]*2 - n · a[n-1] + n*n · a[n-1] - 2*n