docs: def subtract(self, expr, simplify):
          """
          Return a new expression where the `expr` expression has been removed
          from this expression if it exists.
          """
          args = self.args
          if expr in self.args:
              args = list(self.args)
              args.remove(expr)
          elif isinstance(expr, self.__class__):
              if all(arg in self.args for arg in expr.args):
                  args = tuple(arg for arg in self.args if arg not in expr)
          if len(args) == 0:
              return None
          if len(args) == 1:
              return args[0]
  
          newexpr = self.__class__(*args)
          if simplify:
              newexpr = newexpr.simplify()
          return newexpr