docs: def process_wildcard(fractions):
      """
      Processes element with a wildcard ``?`` weight fraction and returns
      composition balanced to 1.0.
      """
      wildcard_zs = set()
      total_fraction = 0.0
      for z, fraction in fractions.items():
          if fraction == '?':
              wildcard_zs.add(z)
          else:
              total_fraction += fraction
  
      if not wildcard_zs:
          return fractions
  
      balance_fraction = (1.0 - total_fraction) / len(wildcard_zs)
      for z in wildcard_zs:
          fractions[z] = balance_fraction
  
      return fractions