#include <stdio.h>

extern double square ( double );   // *** Declare square( ) ***

int main( int argc, char *argv[] )
{
   double a;
   double b;
 
   a = 4;

   b = square( a );  // Declared: **double** square( double ) !!   
 
   printf( "Square of %lf = %lf\n", a, b);
}