realpart and imagpart return the real and imaginary parts of number respectively.
If number is real, then realpart returns number and imagpart returns
(* 0 number), which has the effect that the imaginary part of a rational is
0 and that of a float is a floating-point zero of the same format.
(realpart #c(23 41))
(imagpart #c(23 41.0))
(realpart #c(23 41.0))
Should signal an error of type type-error if number is not a number.