(chance ratio)
  ratio: a flo
  returns a bool
F

Returns #t or #f in the given ratio.

For example, (chance 0.3) will return #t three times out of ten, and return #f seven times out of ten.

When (<= ratio 0.0), the result will always be #f, and when (>= ratio 1.0), the result will always be #t.

coin-flip
rand-select