fix(library/tlean_exporter): add recursion guard (#752)
fix(library/tlean_exporter): add recursion guard (#752)

Fixes a segfault in mathlib due to stack overflow, [reported on Zulip](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Tactic.20regression.20tests.20via.20mathport/near/292324800). (For reasons unknown, this doesn't just throw a stack overflow exception instead of segfaulting: it actually terminates correctly. Does `check_system` do some split stack shenanigans? It seems like it should be disabled.)



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