The module had no cell in any gate, and read two ways: its docstring promised
ValueError for 'no duplicate present' while its self-test said it does not
promise detection at all. Measured at 95e4962: every array of length n+1 over
1..n for n=1..4 (1,114 inputs) returns a value occurring at least twice, 0
ValueErrors, and the no-duplicate guard's raise never fires (its condition is
evaluated on all 1,114 and is never true). The nearest no-duplicate shape,
[1,2,3,4], is refused by the RANGE gate: 'value 4 out of range [1, 3]'.
- find_duplicate.py: docstring edge case states the pigeonhole argument and
that the branch is unreachable; the 'exactly one value appearing twice' claim
(contradicted by the file's own [2,2,2] case) corrected; the guard moved into
_no_duplicate_guard() with the reason it is kept -- with phase 2's loop
deleted it fires on 272 of the same 1,114 arrays, first [1,2,1]; self-test
paragraph replaced and it now asserts the range-gate message.
- test_find_duplicate.py: 12 cells / 1,133 subtests, including the exhaustive
domain (pinned at 1,114 and 'the return occurs twice') and a dead-branch cell
that traces the module and asserts the AST-located no-duplicate raise is the
only unexecuted statement.
- Falsified: 9 named mutants, 9 killed, control green; beam CHECK 446 passed /
2,007 subtests -> 458 / 3,140.