#include <stdio.h>
#include <complex.h>
#undef complex     /* after including complex.h, before declaring struct complex */

//#include <header_that_defines_struct_complex.h>
struct complex
{
    double a;
    double b;
};

int main(void)
{
    // this should all be fine now
    double _Complex z = CMPLX(0.0, -0.0);
    double _Complex z1 = 1.0 + 3.0 * _Complex_I;
    double _Complex z2 = 1.0 - 4.0 * _Complex_I;
    double _Complex z3 = 1.0 + 1.0 * _Imaginary_I;
    double _Complex z4 = 1.0 + 1.0 * _Complex_I;
    double _Complex z5 = 1.0 + 1.0 * I;
    printf("Starting values: Z1 = %.2f + %.2fin", creal(z1), cimag(z1));

    return 0;
}