coerce

static VALUE
rb_big_coerce(VALUE x, VALUE y)
{
    if (FIXNUM_P(y)) {
        return rb_assoc_new(rb_int2big(FIX2LONG(y)), x);
    }
    else if (TYPE(y) == T_BIGNUM) {
       return rb_assoc_new(y, x);
    }
    else {
        rb_raise(rb_eTypeError, "can't coerce %s to Bignum",
                 rb_obj_classname(y));
    }
    /* not reached */
    return Qnil;
}