First Part of A2

In discussion of A2 me and my partner had some split in opinions. He thinks that the children should be a list containing two regexes or strings. And I think that there should be children1 and children2, both as inputs. After that we checked several questions on piazza and decided to use my partner's idea. We found examples extremely helpful. It's hard to come up with examples but after that everything is pretty easy. So we will finished that today and hopefully there's no more problems.
Well what I also found hard:

• to understand what the program __repr__ returns is kind of difficult for us. We have to check the piazza to decide whether or not to let it return exactly same thing with the input value.
• to use property to make the symbol and children read-only.
• to use format to write the __repr__ method. it turns out to be easier to write but harder to think.
• to communicate with partner--- actually more important than come up or writing down codes.