Chapter-4/4-5/4-5.c
/* pop:  pop and return top value from stack */
pop and return top value from stack
pop and return top value from stack
double pop(void) {
  if (sp > 0)
    return val[--sp];
  else {
    printf("error: stack empty\n");
    return 0.0;
  }
}
double pop ( void ) { if ( sp > 0 ) return val [ -- sp ] ; else { printf ( " \n " ) ; return 0.0 ; } }
pop:  pop and return top value from stack
pop : pop and return top value from stack