/usr/lib/ghc-doc/hoogle/erf.txt is in libghc-erf-doc 2.0.0.0-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-erf-doc/html/erf.txt
/usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-erf-doc/html/erf.txt