Ive been taught a counter wich I have managed to get work some times is to put your free hand under the opponents arm and grasp to his colar or shoulder so he cannot straighten he's back to get the choke effective.
This might allso buy you some time if the opponent isnt good with guillotines to get the legs and throw him to the ground and there basicly just pull your head out of the arms.
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No Brain, No Pain!