// Visitor pattern
// some place from the Internet

abstract class ExpressionVisitor 
{ abstract void visitIntExp(IntExp e); 
  abstract void visitAddExp(AddExp e); 
} 

abstract class Expression 
{  abstract void accept(ExpressionVisitor v); 
} 

class IntExp extends Expression 
{ int value; 
  void accept(ExpressionVisitor v)
  { v.visitIntExp(this);
  }
} 

class AddExp extends Expression 
{ Expression e1, e2; 
  void accept(ExpressionVisitor v)
  { v.visitAddExp(this);
  }
}