code: def pauli_sum(*elements: Pauli) -> Pauli:
      
      terms = []
  
      key = itemgetter(0)
      for term, grp in groupby(heapq.merge(*elements, key=key), key=key):
          coeff = sum(g[1] for g in grp)
          if not isclose(coeff, 0.0):
              terms.append((term, coeff))
  
      return Pauli(tuple(terms))
docs: Return the sum of elements of the Pauli algebra