while ( $lastOp && self::$precedence[$token->name] <= self::$precedence[$lastOp->name] ) {
					$this->doOperation( $lastOp, $this->operands );
					array_pop( $this->operators );
					$lastOp = end( $this->operators );
				}

		while ( $op = array_pop( $this->operators ) ) {
			$this->doOperation( $op, $this->operands );
		}

	protected function doOperation( Operator $op ) {
		if ( count( $this->operands ) < 2 ) {
			$op->error( 'missing operand' );
		}
		$right = array_pop( $this->operands );
		$left = array_pop( $this->operands );
		$result = $op->operate( $left, $right );
		$this->operands[] = $result;
	}