external_encoding=

static VALUE set_external_encoding(VALUE self, VALUE encoding)
{
    yaml_parser_t * parser;
    VALUE exception;

    Data_Get_Struct(self, yaml_parser_t, parser);

    if(parser->encoding) {
        exception = rb_const_get_at(mPsych, rb_intern("Exception"));
        rb_raise(exception, "don't set the encoding twice!");
    }

    yaml_parser_set_encoding(parser, NUM2INT(encoding));

    return encoding;
}