There is one of these beasts per INSERT/DELETE/UPDATE statement. It fulfills the contract for the externally visible trigger execution context and it validates that a statement that is about to be executed doesn't violate the restrictions placed upon what can be executed from a trigger.
Note that it is crucial that cleanup() is called once the DML has completed, cleanup() makes sure that users can't do something invalid on a tec reference that they were holding from when the trigger fired.
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