numeric.c:357
static VALUE
flo_minus(x, y)
    VALUE x, y;
{
    switch (TYPE(y)) {
      case T_FIXNUM:
	return rb_float_new(RFLOAT(x)->value - (double)FIX2LONG(y));
      case T_BIGNUM:
	return rb_float_new(RFLOAT(x)->value - rb_big2dbl(y));
      case T_FLOAT:
	return rb_float_new(RFLOAT(x)->value - RFLOAT(y)->value);
      default:
	return rb_num_coerce_bin(x, y);
    }
}
