/usr/share/X11/locale/pt_PT.UTF-8/Compose is in libx11-data 2:1.6.4-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | include "/usr/share/X11/locale/en_US.UTF-8/Compose"
<dead_acute> <C> : "Ç" Ccedilla # LATIN CAPITAL LETTER C WITH CEDILLA
<dead_acute> <c> : "ç" ccedilla # LATIN SMALL LETTER C WITH CEDILLA
|