/usr/include/ is in libbullet-extras-dev 2.81-rev2613+dfsg2-1.
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/usr/include/bullet/ConvexDecomposition/
/usr/include/bullet/BulletFileLoader/
/usr/include/bullet/
/usr/include/bullet/ConvexDecomposition/meshvolume.h
/usr/include/bullet/ConvexDecomposition/bestfitobb.h
/usr/include/bullet/ConvexDecomposition/float_math.h
/usr/include/bullet/ConvexDecomposition/splitplane.h
/usr/include/bullet/ConvexDecomposition/bestfit.h
/usr/include/bullet/ConvexDecomposition/cd_vector.h
/usr/include/bullet/ConvexDecomposition/planetri.h
/usr/include/bullet/ConvexDecomposition/cd_wavefront.h
/usr/include/bullet/ConvexDecomposition/fitsphere.h
/usr/include/bullet/ConvexDecomposition/ConvexDecomposition.h
/usr/include/bullet/ConvexDecomposition/cd_hull.h
/usr/include/bullet/ConvexDecomposition/concavity.h
/usr/include/bullet/ConvexDecomposition/vlookup.h
/usr/include/bullet/ConvexDecomposition/raytri.h
/usr/include/bullet/ConvexDecomposition/ConvexBuilder.h
/usr/include/bullet/GIMPACTUtils/
/usr/include/bullet/GIMPACTUtils/btGImpactConvexDecompositionShape.h
/usr/include/bullet/HACD/
/usr/include/bullet/HACD/hacdCircularList.inl
/usr/include/bullet/HACD/hacdVector.h
/usr/include/bullet/HACD/hacdHACD.h
/usr/include/bullet/HACD/hacdVersion.h
/usr/include/bullet/HACD/hacdCircularList.h
/usr/include/bullet/HACD/hacdGraph.h
/usr/include/bullet/HACD/hacdManifoldMesh.h
/usr/include/bullet/HACD/hacdICHull.h
/usr/include/bullet/HACD/hacdVector.inl
/usr/include/bullet/BulletWorldImporter/
/usr/include/bullet/BulletWorldImporter/btWorldImporter.h
/usr/include/bullet/BulletWorldImporter/btBulletWorldImporter.h
/usr/include/bullet/BulletXmlWorldImporter/
/usr/include/bullet/BulletXmlWorldImporter/string_split.h
/usr/include/bullet/BulletXmlWorldImporter/btBulletXmlWorldImporter.h
/usr/include/bullet/BulletFileLoader/bDNA.h
/usr/include/bullet/BulletFileLoader/bDefines.h
/usr/include/bullet/BulletFileLoader/bChunk.h
/usr/include/bullet/BulletFileLoader/bCommon.h
/usr/include/bullet/BulletFileLoader/autogenerated/
/usr/include/bullet/BulletFileLoader/autogenerated/bullet.h
/usr/include/bullet/BulletFileLoader/btBulletFile.h
/usr/include/bullet/BulletFileLoader/bFile.h