docs: def _tables_used(self):
          """
          Tables injected into the step.
  
          Returns
          -------
          tables : set of str
  
          """
          args = list(self._argspec.args)
          if self._argspec.defaults:
              default_args = list(self._argspec.defaults)
          else:
              default_args = []
          # Combine names from argument names and argument default values.
          names = args[:len(args) - len(default_args)] + default_args
          tables = set()
          for name in names:
              parent_name = name.split('.')[0]
              if is_table(parent_name):
                  tables.add(parent_name)
          return tables