docs: def _escape(self, bits):
          """
          value: 'foobar {' -> 'foobar {{'
          value: 'x}' -> 'x}}'
          """
          # for value, field_name, format_spec, conversion in bits:
          while True:
              try:
                  value, field_name, format_spec, conversion = next(bits)
                  if value:
                      end = value[-1]
                      if end in (u'{', u'}'):
                          value += end
                  yield value, field_name, format_spec, conversion
              except StopIteration:
                  break