DeepSMILES avoids the use of paired parentheses by transforming to a syntax related to Reverse Polish Notation (RPN) , a parenthesis-free syntax used to describe arithmetic expressions. For example, an equivalent RPN expression for "2 x (4 + 6) + 3 x (7 -5) =" would be "2 4 6 + x 3 7 5 -x + =". This expression is evaluated by placing each successive digit onto a stack; when an operator is reached, it is applied to the top two items on the stack which are then replaced by the result.