Def sumdeq(a;b)(p,q)
Def == InjCase(p; pa. InjCase(q; qa. 1of(a)(pa,qa); qb. false); pb.
Def == InjCase(q; qa. false; qb. 1of(b)(pb,qb)))

is mentioned by

Thm* A,B:Type, a:EqDecider(A), b:EqDecider(B). Thm* sum-deq(A;B;a;b)  (p,q:A+B. p = q  sumdeq(a;b)(p,q))	[sum-deq_wf]
Thm* a:EqDecider(A), b:EqDecider(B), p,q:A+B. p = q  sumdeq(a;b)(p,q)	[sumdeq-property]
Def union-deq(A;B;a;b) == <sumdeq(a;b),sum-deq(A;B;a;b)>	[union-deq]

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