code: def split_input(self, input_filepath):
          
          with open(input_filepath, 'r') as input_file:
              input_file_str = input_file.read()
              merge_file_str, parsetree_str = input_file_str.split('ParentedTree', 1)
              return merge_file_str, 'ParentedTree' + parsetree_str
docs: Splits the input file into the 'merge file' (which contains
          the EDUs) and the 'parsetree file'.