Climbing to the least

Naive evaluation starts from {} and only adds a fact if there is a rule for it:

• even(0) is a fact, which leads to odd(1), which leads to even(2), …

  Every new fact traces back to even(0).

• odd(0) can only come from even(1), which can only come from odd(0), …

  Nothing outside the cycle forces it, so starting from {} never adds it.

If naive evaluation terminates, it gives us the least fixed point.