/usr/share/texlive/texmf-dist/metapost/support/charlib/bx is in texlive-metapost 2014.20141024-2+deb8u1.
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1 2 3 4 5 6 7 8 | % $Id: bx,v 1.2 2004/09/19 21:47:11 karl Exp $
% Public domain.
vardef C.bx(expr n) =
save p; picture p;
p=nullpicture;
addto p contour ((0,0)--(1,0)--(1,1)--(0,1)--cycle)
scaled (fontsize n/2);
p enddef;
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