Returns True if `expr` is a subtree.

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