Five operators, +, -, *, /, ^.
The +, -, *, and / operators should be left associative.
The ^ operator should be right associative.
The ^ operator should have highest precedence and +,- should have the lowest precedence.


    Exp ::= Exp '+' Term     // left recursion means left associative
          | Exp '-' Term
          | Term

   Term ::= Term '*' Factor
          | Term '/' Factor
          | Factor

 Factor ::= Base '^' Factor  // right recursion means right associative
          | Base

   Base ::= '(' Exp ')'
          | Integer
          | Variable


Remove the left recursion so that we can write a recursive descent parser.

   Exp ::= Term ( '+' Term | '-' Term )*

  Term ::= Factor ( '*' Factor | '/' Factor )*

Factor ::= Base [ '^' Factor ]

  Base ::= '(' Exp ')'
         | Integer
         | Variable