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Proof step 12 from proof validrefnotnull-union-at-right (
previous step
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⊦ is_obj(str, r, st) ↔ class+∃(str, st[r -- _type] .type .class) ∧ ¬ is_abstract(st[r -- _type] .type .class) ∧ is_obj(r, st)
A spec-lemma