#include <stdio.h>

struct complex {double x,y; };

void mult(struct complex   op1,
	  struct complex   op2,
	  struct complex * result)
{
  result->x = op1.x * op2.x
            - op1.y * op2.y;
  result->y = op1.x * op2.y
            + op1.y * op2.x;
}

main()
{
  struct complex a = {1.0, 3.0};

  mult(a,a,&a);

  printf("a*a = %e + %e*i\n",
	        a.x, a.y);
}