- construct expression tree from postfix expression (with stack)
- postfix expression from in-order traversal of expression tree
- simplify expression tree (https://docs.sympy.org/latest/tutorials/intro-tutorial/simplification.html):
- cancel
- factor
- collect
- expand
- partial fraction decomposition
- trig
- powers
- exponentials
- logarithms
- mapping variables in expression tree
- (a-b) -> +,a,*,-1,b
- associtativity and commutativity
- level indicates operator precedence (operations at lower levels are evaluated later)