Some definitions of interest.
bi-graph	Def bi-graph(G;to;from) Def == i:|G|.  Def == (lto(i).destination(l) = i Def == & (G(source(l))) Def == & (l  from(source(l))) Def == & (lnk-inv(l)  from(i))) Def == & (lfrom(i).source(l) = i Def == & & (G(destination(l))) Def == & & (l  to(destination(l))) Def == & & (lnk-inv(l)  to(i)))
spanner	Def spanner(f;T;to;from) Def == (l:Edge(T). f(l) = f(inverse(l))) Def == & (i:|T|, l1,l2:Edge(T). Def == & ((l1  to(i)) Def == & ( Def == & ((l2  to(i))  l1 = l2  IdLnk  (f(l1))  (f(l2)))
	Thm* T:(Id), to,from:(|T|(IdLnk List)), f:(Edge(T)). Thm* bi-graph(T;to;from)  spanner(f;T;to;from)  Prop
bi-graph-edge	Def Edge(G) == {l:IdLnk| i:|G|. (l  from(i)) }
IdLnk	Def IdLnk == IdId
	Thm* IdLnk  Type
rset	Def |R| == {i:Id| (R(i)) }
	Thm* R:(Id). |R|  Type
Id	Def Id == Atom
	Thm* Id  Type

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