class Operator {
  private char _op;
  public Operator (char c) { _op = c; }
}

abstract class Expression {} // Expression = Value + Variable + Binary

class Value extends Expression { // Value = int
  int intValue;
}
class Variable extends Expression { // Variable = String
  String name;
}
class Binary extends Expression { //@{}@XPCL{Binary = Expression @Sym {multiply} Operator @Sym {multiply} Expression}
  Operator op;
  Expression term1, term2;
}