protected function _operator($key, $value, array $schema = [], array $options = []) {
		$defaults = ['boolean' => 'AND'];
		$options += $defaults;

		$op = strtoupper(key($value));
		$value = current($value);

		$config = $this->_operators[$op];
		$key = $this->name($key);
		$values = [];

		if (!is_object($value)) {
			if ($value === null) {
				$value = [null];
			}
			foreach ((array) $value as $val) {
				$values[] = $this->value($val, $schema);
			}
		} elseif (isset($value->scalar)) {
			return "{$key} {$op} {$value->scalar}";
		}

		switch (true) {
			case (isset($config['format'])):
				return $key . ' ' . Text::insert($config['format'], $values);
			case (is_object($value) && isset($config['multiple'])):
				$op = $config['multiple'];
				$value = trim(rtrim($this->renderCommand($value), ';'));
				return "{$key} {$op} ({$value})";
			case (count($values) > 1 && isset($config['multiple'])):
				$op = $config['multiple'];
				$values = join(', ', $values);
				return "{$key} {$op} ({$values})";
			case (count($values) > 1):
				return join(" {$options['boolean']} ", array_map(
					function($v) use ($key, $op) { return "{$key} {$op} {$v}"; }, $values
				));
		}
		return "{$key} {$op} {$values[0]}";
	}