/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/ is in libphobos-5-dev 5.5.0-12ubuntu1.
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..
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/bits.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/config.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/gc.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/os.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/pooltable.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/proxy.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gc/stats.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/atomics.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/attribute.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/backtrace.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/builtins.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/config.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/deh.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/libbacktrace.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/unwind/
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/unwind/arm.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/unwind/generic.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/unwind/package.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcc/unwind/pe.d
/usr/lib/gcc/x86_64-linux-gnu/5/include/d/gcstub/gc.d