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

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

building blocks

`{x in R| x * 2}`

3.3.4 Theorem [Principles of Induction on Terms]

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.