theorem
  FinUnion({.i,j,k.},f) = f.i \/ f.j \/ f.k
proof
  FinUnion A is idempotent & FinUnion A is commutative by Th34,Th35;
  hence FinUnion({.i,j,k.},f) = FinUnion A.(FinUnion A.(f.i, f.j), f.k) by Th16
,Th36
    .= FinUnion A.(f.i \/ f.j, f.k) by Def4
    .= f.i \/ f.j \/ f.k by Def4;
end;
