/usr/lib/x86_64-linux-gnu/glibmm-2.4/proc/m4/initialize_gio.m4 is in libglibmm-2.4-dev 2.42.0-1.
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dnl Initializations for giomm C++ types from Gio C types.
dnl
dnl UnixFDList
_INITIALIZATION(`Glib::RefPtr<UnixFDList>&',`GUnixFDList*', `$3 = Glib::wrap($4)')
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