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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def is_in(self, expr):
 """Returns True if `expr` is a subtree."""

    if expr == self:
      return True
    is_in_left = is_in_expr(self.left, expr)
    is_in_right = is_in_expr(self.right, expr)
    return is_in_left or is_in_right