code: private SemanticType checkIntegerOperator(Expr.BinaryOperator expr, Environment environment) {
  		checkOperand(Type.Int, expr.getFirstOperand(), environment);
  		checkOperand(Type.Int, expr.getSecondOperand(), environment);
  		return Type.Int;
  	}
docs: Check the type for a given arithmetic operator. Such an operator has the type
  int, and all children should also produce values of type int.
  
  @param expr
  @return