reserve x,y,z,X,Y for set;
reserve X,Y for non empty set,
  f for Function of X,Y;
reserve X, Y for non empty set,
  F for (BinOp of Y),
  B for (Element of Fin X),
  f for Function of X,Y;
reserve A for set,
  x,y,z for Element of Fin A;
reserve X,Y for non empty set,
  A for set,
  f for (Function of X, Fin A),
  i,j,k for (Element of X);

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;
