START TRANSACTION IF NOT EXISTS

SHORT VERSION: Could we have a START TRANSACTION IF EXISTS that does not commit existing transaction?

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LONG VERSION:

Sometimes, when a procedure is called, there are possible 2 cases:
1) The procedure has been called by a trigger: I want to continue the active transaction.
2) Called by the cli, I want to be sure that a transaction exists.

But START TRANSACTION commits existing transaction. A snippet like this does NOT solve the problem:

IF NOT @@in_transaction THEN

		SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;

		START TRANSACTION;

END IF;

Because, if the procedure is called by a trigger, I get:
ERROR 1422 (HY000): Explicit or implicit commit is not allowed in stored function or trigger.

START TRANSACTION IF EXISTS would solve the problem.