complex.h

https://www.geeksforgeeks.org/complex-h-header-file-in-c-with-examples/

Macro Name	Expands To
complex	_Complex
imaginary	_Imaginary
_Complex_I	(const float _Complex) i
_Imaginary_I	(const float _Imaginary) i
I	_Imaginary_I(_Complex_I if _Imaginary_I is absent)