bignum.c:1719
static VALUE
rb_big_coerce(x, y)
    VALUE x, y;
{
    if (FIXNUM_P(y)) {
	return rb_assoc_new(rb_int2big(FIX2LONG(y)), x);
    }
    else {
	rb_raise(rb_eTypeError, "Can't coerce %s to Bignum",
		 rb_obj_classname(y));
    }
    /* not reached */
    return Qnil;
}
