title: Adding missing subtraction and division operators to sympy
body: <p>I am trying to implement step by step math solver in sympy. Arithmetic operators +, -, *, /, ** are mapped to combinations of three core SymPy’s classes: Add, Mul and Pow. 
  However I need a substraction and division in expression tree so that I can print it while stepping through. I know that I need to change or at least override some behavior in sympy but I dont know where and how?
  Any idea on this? Thanks.</p>