anchor.diff:        functor = (defaultFunctor name) // { wrapped = elemType; };
       };
   
  +    # Null or value of ...
       nullOr = elemType: mkOptionType rec {
         name = "nullOr";
         description = "null or ${elemType.description}";
  
positive.code:       functor = (defaultFunctor name) // { wrapped = elemType; };
      };
  
      uniq = elemType: mkOptionType rec {
        name = "uniq";
        inherit (elemType) description check;
  
negative.code:         functor = (defaultFunctor name) // { payload = values; binOp = a: b: unique (a ++ b); };
        };
  
      # Either value of type `t1` or `t2`.
      either = t1: t2: mkOptionType rec {
        name = "either";
        description = "${t1.description} or ${t2.description}";
  