/usr/include/hfst/implementations/ is in libhfst42-dev 3.9.0~r4595-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/hfst/implementations/ComposeIntersectFst.h
/usr/include/hfst/implementations/ComposeIntersectLexicon.h
/usr/include/hfst/implementations/ComposeIntersectRule.h
/usr/include/hfst/implementations/ComposeIntersectRulePair.h
/usr/include/hfst/implementations/ComposeIntersectUtilities.h
/usr/include/hfst/implementations/ConvertTransducerFormat.h
/usr/include/hfst/implementations/FomaTransducer.h
/usr/include/hfst/implementations/HfstFastTransitionData.h
/usr/include/hfst/implementations/HfstOlTransducer.h
/usr/include/hfst/implementations/HfstTransition.h
/usr/include/hfst/implementations/HfstTransitionGraph.h
/usr/include/hfst/implementations/HfstTropicalTransducerTransitionData.h
/usr/include/hfst/implementations/LogWeightTransducer.h
/usr/include/hfst/implementations/SfstTransducer.h
/usr/include/hfst/implementations/TropicalWeightTransducer.h
/usr/include/hfst/implementations/XfsmTransducer.h
/usr/include/hfst/implementations/compose_intersect/
/usr/include/hfst/implementations/compose_intersect/ComposeIntersectFst.h
/usr/include/hfst/implementations/compose_intersect/ComposeIntersectLexicon.h
/usr/include/hfst/implementations/compose_intersect/ComposeIntersectRule.h
/usr/include/hfst/implementations/compose_intersect/ComposeIntersectRulePair.h
/usr/include/hfst/implementations/compose_intersect/ComposeIntersectUtilities.h
/usr/include/hfst/implementations/optimized-lookup/
/usr/include/hfst/implementations/optimized-lookup/convert.h
/usr/include/hfst/implementations/optimized-lookup/pmatch.h
/usr/include/hfst/implementations/optimized-lookup/transducer.h