z2 := new(Rat).SetFloat64(1.25)   // z2 := 5/4
	z3 := new(Float).SetInt(z1)       // z3 := 123.0

Setters, numeric operations and predicates are represented as methods of
the form:

	func (z *T) SetV(v V) *T          // z = v
	func (z *T) Unary(x *T) *T        // z = unary x
	func (z *T) Binary(x, y *T) *T    // z = x binary y
	func (x *T) Pred() P              // p = pred(x)