/usr/include/osl/checkmate/ is in libosl-dev 0.8.0-1.4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/osl/checkmate/checkmateIfCapture.h
/usr/include/osl/checkmate/dfpn.h
/usr/include/osl/checkmate/dfpnParallel.h
/usr/include/osl/checkmate/dfpnRecord.h
/usr/include/osl/checkmate/disproofPieces.h
/usr/include/osl/checkmate/dualDfpn.h
/usr/include/osl/checkmate/fixedDepthSearcher.h
/usr/include/osl/checkmate/fixedDepthSearcher.tcc
/usr/include/osl/checkmate/fixedDepthSolverExt.h
/usr/include/osl/checkmate/immediateCheckmate.h
/usr/include/osl/checkmate/immediateCheckmate.tcc
/usr/include/osl/checkmate/immediateCheckmateTable.h
/usr/include/osl/checkmate/libertyEstimator.h
/usr/include/osl/checkmate/oracleAdjust.h
/usr/include/osl/checkmate/pawnCheckmateMoves.h
/usr/include/osl/checkmate/pieceCost.h
/usr/include/osl/checkmate/proofDisproof.h
/usr/include/osl/checkmate/proofNumberTable.h
/usr/include/osl/checkmate/proofPieces.h
/usr/include/osl/checkmate/proofPiecesUtil.h
/usr/include/osl/checkmate/proofTreeDepthDfpn.h