.Number: ClassType
.Complex: ClassType
.Complex <: .Number
.Complex.
    real: Float
    imag: Float
    conjugate: |C <: .Complex|(self: C) -> C
.Real: ClassType
.Real <: .Complex
.Rational: ClassType
.Rational <: .Real
.Rational.
    numerator: Int
    denominator: Int
.Integral: ClassType
.Integral <: .Rational