code: def fork(self, strictindex, new_value):
          
          forked_chunk = YAMLChunk(
              deepcopy(self._ruamelparsed),
              pointer=self.pointer,
              label=self.label,
              key_association=copy(self._key_association),
          )
          forked_chunk.contents[self.ruamelindex(strictindex)] = new_value.as_marked_up()
          forked_chunk.strictparsed()[strictindex] = deepcopy(new_value.as_marked_up())
          return forked_chunk
docs: Return a chunk referring to the same location in a duplicated document.
  
          Used when modifying a YAML chunk so that the modification can be validated
          before changing it.