WWhen2 `a != 0`, there are two solutions to `ax^2 + bx + c = 0` and they are2

`x = (-b +- sqrt(b^2-4ac))/(2a) .`

Induction on depth: If, for each term s, given P(r) for all r such that depth(r) < depth(s) we can show P(s), then P(s) holds for all s./section>