cis radians => number
radians---a real.
number---a complex.
cis returns the value of~e^{i\cdot radians}, which is a complex in which the real part is equal to the cosine of radians, and the imaginary part is equal to the sine of radians.
(cis 0) => #C(1.0 0.0)
section Rule of Float Substitutability
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