docs: def _pvar_to_name(cls, pvar_expr):
          '''Returns the name of pvariable.
  
          Returns:
              Name of pvariable.
          '''
          functor = pvar_expr[0]
          arity = len(pvar_expr[1]) if pvar_expr[1] is not None else 0
          return '{}/{}'.format(functor, arity)