/usr/ is in ruby2.5-dev 2.5.1-1ubuntu1.
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..
/usr/lib/
/usr/lib/x86_64-linux-gnu/
/usr/lib/x86_64-linux-gnu/pkgconfig/
/usr/include/
/usr/include/ruby-2.5.0/
/usr/include/ruby-2.5.0/ruby/
/usr/include/ruby-2.5.0/ruby/backward/
/usr/include/ruby-2.5.0/ruby/backward/classext.h
/usr/include/ruby-2.5.0/ruby/backward/rubyio.h
/usr/include/ruby-2.5.0/ruby/backward/rubysig.h
/usr/include/ruby-2.5.0/ruby/backward/st.h
/usr/include/ruby-2.5.0/ruby/backward/util.h
/usr/include/ruby-2.5.0/ruby/backward.h
/usr/include/ruby-2.5.0/ruby/debug.h
/usr/include/ruby-2.5.0/ruby/defines.h
/usr/include/ruby-2.5.0/ruby/digest.h
/usr/include/ruby-2.5.0/ruby/encoding.h
/usr/include/ruby-2.5.0/ruby/intern.h
/usr/include/ruby-2.5.0/ruby/io.h
/usr/include/ruby-2.5.0/ruby/missing.h
/usr/include/ruby-2.5.0/ruby/onigmo.h
/usr/include/ruby-2.5.0/ruby/oniguruma.h
/usr/include/ruby-2.5.0/ruby/re.h
/usr/include/ruby-2.5.0/ruby/regex.h
/usr/include/ruby-2.5.0/ruby/ruby.h
/usr/include/ruby-2.5.0/ruby/st.h
/usr/include/ruby-2.5.0/ruby/subst.h
/usr/include/ruby-2.5.0/ruby/thread.h
/usr/include/ruby-2.5.0/ruby/thread_native.h
/usr/include/ruby-2.5.0/ruby/util.h
/usr/include/ruby-2.5.0/ruby/version.h
/usr/include/ruby-2.5.0/ruby/vm.h
/usr/include/ruby-2.5.0/ruby.h
/usr/include/x86_64-linux-gnu/
/usr/include/x86_64-linux-gnu/ruby-2.5.0/
/usr/include/x86_64-linux-gnu/ruby-2.5.0/ruby/
/usr/include/x86_64-linux-gnu/ruby-2.5.0/ruby/config.h
/usr/lib/x86_64-linux-gnu/pkgconfig/ruby-2.5.pc
/usr/share/
/usr/share/doc/
/usr/share/doc/ruby2.5-dev/
/usr/share/doc/ruby2.5-dev/copyright
/usr/lib/x86_64-linux-gnu/libruby-2.5.so
/usr/share/doc/ruby2.5-dev/changelog.Debian.gz