aha! I figured it out:
let function F(z) srand(z) return rnd()<0.1 end. we don’t particularly know (or care) which specific integers F(z) is true for, but for any given ‘a’, ‘e’, and z=flr(a*99+e) where F(z) is true, then increasing ‘e’ slightly and decreasing ‘a’ slightly will still give the same value z as before, so F(z) is still true.
and in the context of this cart, ‘e’ increases over time, and x decreases as ‘a’ decreases, so the x-values that clouds are drawn at move left over time. cool!