docs: def _lval_add_towards_polarity(x, polarity):
      """Compute the appropriate Lval "kind" for the limit of value `x` towards
      `polarity`. Either 'toinf' or 'pastzero' depending on the sign of `x` and
      the infinity direction of polarity.
  
      """
      if x < 0:
          if polarity < 0:
              return Lval('toinf', x)
          return Lval('pastzero', x)
      elif polarity > 0:
          return Lval('toinf', x)
      return Lval('pastzero', x)