mb event system 7 Sections EventSystems Doc
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Def p(i) == source(in(i))

is mentioned by

Thm* R:(Id), in,out:(|R|IdLnk), i,j:|R|.
Thm* ring(R;in;out)  i = p(j)  d(i;p(j)) = d(i;j)-1		[rdist-rprev]
Thm* R:(Id), in,out:(|R|IdLnk), j:|R|. ring(R;in;out)  n(p(j)) = j[rnext-rprev]

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mb event system 7 Sections EventSystems Doc