Make the Reverse Polish Notation calculator work with variables

  If the input is
    2 3 4 + * -> a =
  that means work out 2*(3+4) and remember that as the value of a.
  -> needs to be handled specially, don't treat it as a normal operator

  If the input is
    2 a * =
  then the result should be whatever 2 * a is

  You should define a struct/object to represent the calculator's
  memory. As well as a constructor, it should have these three methods:

    void set(string variablename, double value);
    bool isdefined(string variablename);
    double getvalue(string variablename);

  There should be no limit on the number of variables that your memory
  can store.