public function groupBy( array $expressions )  {
		foreach ( $expressions as $expression ) {
			$this->expressionValidator->validate( $expression,
				ExpressionValidator::VALIDATE_VARIABLE | ExpressionValidator::VALIDATE_FUNCTION_AS
			);
		}

		$expression = implode( ' ', $expressions );
		$this->usageValidator->trackUsedPrefixes( $expression );
		$this->usageValidator->trackUsedVariables( $expression );
		$this->modifiers['GROUP BY'] = $expression;
	}