def make_charge
      if !todo.annotated?
        "Missing due date annotation"
      elsif todo.due_date.overdue? && todo.tag?
        "Overdue due date #{todo.due_date.to_date} via tag"
      elsif todo.due_date.overdue?
        "Overdue due date"
      end
    end
def make_charge
      if !todo.annotated?
        "Missing due date annotation"
      elsif todo.due_date.overdue? && todo.tag?
        "Overdue due date #{todo.due_date.to_date} via tag"
      elsif todo.due_date.overdue?
        "Overdue due date"
      end
    end
What is the problem with this todo?

 @return [String] if there's a problem
 @return [NilClass] if no charge needed
 @api private