e
analog to <code><a href="refO.html#or/2">or/2</a></code>, as each of them
generates a sequence of values. However, the generator clauses behave different,
as they will not generate an exhaustive set of values upon backtracking, one
after the other, where the next gets its turn when the previous one is
exhausted. Instead, all clauses will generate their values quasi-parallel, with
a built-in optimization so that successful clauses will be called with a higher
probability. "Successful" means that the return