/usr/share/texmf-texlive/metapost/support/charlib/ao.x is in texlive-metapost 2009-15.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | % $Id: ao,v 1.2 2004/09/19 21:47:11 karl Exp $
% Public domain.
vardef C.ao(expr n) =
save p,q; picture p,q;
p="a"infont n;
q=char202 infont n;
addto p also q
shifted(xpart(urcorner p-urcorner q)/2,.05*fontsize n);
p enddef;
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