FlooP
FlooP (named for free loops) is an extension of the programming language BlooP, both presented in Gödel, Escher, Bach: An eternal golden braid. FlooP augments BlooP with a single feature: MU-LOOPS, relaxing the requirement that the number of steps in a loop must be known ahead of time.
MU-LOOP:
BLOCK n: BEGIN
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BLOCK n: END