theorem
  FinUnion({.i,j.},f) = f.i \/ f.j
proof
  FinUnion A is idempotent & FinUnion A is commutative by Th34,Th35;
  hence FinUnion({.i,j.},f) = FinUnion A.(f.i, f.j) by Th15,Th36
    .= f.i \/ f.j by Def4;
end;
