λS.λl1,l2.λw:word S.∃w1,w2.w1 @ w2 = w ∧ l1 w1 ∧ l2 w2.
interpretation "cat lang" 'pc a b = (cat ? a b).
+(* BEGIN HERE
+
definition star ≝ λS.λl.λw:word S.∃lw.flatten ? lw = w ∧ conjunct ? lw l.
interpretation "star lang" 'pk l = (star ? l).
| #x; #y; #z; #w; #a; #b; #c; #d; #K; ncases (?:False); /2/ ]
##| #r1; #r2; #r1'; #r2'; #H1; #H2; #H3; #H4; #H5; #H6;
+*)
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