code: def proximal(self):
          
  
          domain = self.domain
  
          class ProximalSum(Operator):
              """Proximal operator."""
  
              def __init__(self, sigma):
                  self.sigma = sigma
                  super(ProximalSum, self).__init__(
                      domain=domain, range=domain, linear=False)
  
              def _call(self, x, out):
  
                  offset = 1 / x.size * (self.sum_value - x.ufuncs.sum())
                  out.assign(x)
                  out += offset
  
          return ProximalSum
docs: Return the `proximal factory` of the functional.