OpenSSL::BN#coerce

static VALUE
ossl_bn_coerce(VALUE self, VALUE other)
{
    switch(TYPE(other)) {
    case T_STRING:
        self = ossl_bn_to_s(0, NULL, self);
        break;
    case T_FIXNUM:
    case T_BIGNUM:
        self = ossl_bn_to_i(self);
        break;
    default:
        if (!RTEST(rb_obj_is_kind_of(other, cBN))) {
            ossl_raise(rb_eTypeError, "Don't know how to coerce");
        }
    }
    return rb_assoc_new(other, self);
}