code: def union(self, other):
          
          if isinstance(other, tuple):
              other = list(other)
          return type(self)(super().__add__(other))
docs: Returns a FrozenList with other concatenated to the end of self.
  
          Parameters
          ----------
          other : array-like
              The array-like whose elements we are concatenating.
  
          Returns
          -------
          diff : FrozenList
              The collection difference between self and other.