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
def is_in ( self , expr ) : 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
Returns True if `expr` is a subtree.