Who Cites w-isnull?
w-isnull	Def isnull(a) == isl(a)
isl	Def isl(x) == InjCase(x; y. true; z. false)
	Thm* A,B:Type, x:A+B. isl(x)

Syntax:

isnull(a)

has structure:

w-isnull(w; a)

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