Some definitions of interest.
da-outlinks	Def da-outlinks(da;i) Def == mapfilter(k.da-outlink-f(da;k);k.has-src(i;k);fpf-dom-list(da))
IdLnk	Def IdLnk == IdId
	Thm* IdLnk  Type
Id	Def Id == Atom
	Thm* Id  Type
fpf-empty	Def  == <nil,x.>
l_member	Def (x  l) == i:. i<||l|| & x = l[i]  T
	Thm* T:Type, x:T, l:T List. (x  l)  Prop
mapfilter	Def mapfilter(f;P;L) == map(f;filter(P;L))
	Thm* T:Type, P:(T), T':Type, f:({x:T| P(x) }T'), L:T List. Thm* mapfilter(f;P;L)  T' List

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