docs: def charge_sign(self):
          """Charge sign text"""
          if self.charge > 0:
              sign = "+"
          elif self.charge < 0:
              sign = "–"  # en dash, not hyphen-minus
          else:
              return ""
          ab = abs(self.charge)
          if ab > 1:
              return str(ab) + sign
          return sign