/**
   * Checks if expr is a valid infix notation expression
   * @param expr
   * @return
   */
  public static TokenQueue checkInfix(TokenStacksQueue expr){
	  int parens = 0;
	  Token token;
	  int operatorflag = 0;  //0 after operator, 1 after operand
	  int operandflag = 1;   //0 after operand, 1 after operator
	  
	  for(int i = 0; i < expr.size(); i++){
		  token = expr.dequeue();
		  
		  //  Parenthesis
		  if(token.getType() == 3){
			  Parenthesis paren = (Parenthesis)token;
			  if(paren.isLeft()) parens++;
			  else parens--;
		  }
		  
		  //  Operator
		  else if(token.getType() == 2){
			  if(operatorflag == 0){
				  throw new RuntimeException("Error: invalid infix notation");
			  } 
			  else 
			  operatorflag = 0;
			  operandflag = 1;
		  }
		  
		  //  Operand
		  else if(token.getType() == 1){
			  if(operandflag == 0){
				  throw new RuntimeException("Error: invalid infix notation");
			  }
			  else
			  operatorflag = 1;
			  operandflag = 0;
		  }
		  
		  expr.enqueue(token);
	  }
	  if(parens == 0) return expr;
	  else throw new RuntimeException("Error: invalid infix notation");
  }