code: function maybeModifier (predicate) {
      var modifiedPredicate = function () {
        if (not.assigned(arguments[0])) {
          return true;
        }
  
        return predicate.apply(null, arguments);
      };
      modifiedPredicate.l = predicate.length;
  
      // Hackishly indicate that this is a maybe.xxx predicate.
      // Without this flag, the alternative would be to iterate
      // through the maybe predicates or use indexOf to check,
      // which would be time-consuming.
      modifiedPredicate.m = true;
  
      return modifiedPredicate;
    }
docs: Public modifier `maybe`.
  
  Returns true if predicate argument is  null or undefined,
  otherwise propagates the return value from `predicate`.