docs: def locateChild(self, ctx, segments):
          """
          Return a Deferred which will fire with the customized version of the
          resource being located.
          """
          D = defer.maybeDeferred(
              self.currentResource.locateChild, ctx, segments)
  
          def finishLocating((nextRes, nextPath)):
              custom = ixmantissa.ICustomizable(nextRes, None)
              if custom is not None:
                  return (custom.customizeFor(self.forWho), nextPath)
              self.currentResource = nextRes
              if nextRes is None:
                  return (nextRes, nextPath)
              return (_CustomizingResource(nextRes, self.forWho), nextPath)
  
          return D.addCallback(finishLocating)