Chapter 5: structural numbers
   - the imaginary number is the constant "im"
   julia> (1 - 3im) / (2 + 2im)
   -0.5 - 1.0im
   julia> real(1 + 2im) # real part of z
   1
   julia> imag(1 + 2im) # imaginary part of z
   2
   julia> conj(1 + 2im) # complex conjugate of z
   1 - 2im
   julia> abs(1 + 2im) # absolute value of z
   2.23606797749979
   julia> abs2(1 + 2im) # squared absolute value
   5
   julia> angle(1 + 2im) # phase angle in radians
   1.
   julia> sqrt(-1)
   ERROR: DomainError with -1.0:
   sqrt will only return a complex result if called with a complex
   argument. Try sqrt(Complex(x)).
   Stacktrace:
   [...]
   julia> sqrt(-1 + 0im)
   0.0 + 1.0im
   constructing a complex number at runtime:
   julia> a = 1; b = 2; complex(a, b)
   1 + 2im
   Rationals are constructed using the // operator:
   julia> 2//3
   2//3
   julia> -4//-12
   1//3
   julia> numerator(2//3)
   2
   julia> denominator(2//3)
   3
   julia> float(3//4)
   0.75
   julia> x = -3//0
   -1//0