code: def commatize(leafs):
      
      new_leafs = []
      for leaf in leafs:
          new_leafs.append(leaf)
          new_leafs.append(Comma())
      del new_leafs[-1]
      return new_leafs
docs: Accepts/turns: (Name, Name, ..., Name, Name)
      Returns/into: (Name, Comma, Name, Comma, ..., Name, Comma, Name)