code: def _replace_placeholder(self, string, custom_id):
          
          if string is None:
              return None
          self.log([u"Replacing '%s' with '%s' in '%s'", gc.PPV_OS_TASK_PREFIX, custom_id, string])
          return string.replace(gc.PPV_OS_TASK_PREFIX, custom_id)
docs: Replace the prefix placeholder
          :class:`~aeneas.globalconstants.PPV_OS_TASK_PREFIX`
          with ``custom_id`` and return the resulting string.
  
          :rtype: string