/usr/lib/petscdir/3.1/makefile.html is in petsc3.1-doc 3.1.dfsg-11ubuntu1.
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It is a symlink to /usr/lib/petscdir/3.1/../../../share/doc/petsc3.1-doc/makefile.html
/usr/lib/petscdir/3.1/../../../share/doc/petsc3.1-doc/makefile.html