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;

theorem Th35:
  FinUnion A is commutative
proof
  let x,y;
  thus FinUnion A.(x,y) = y \/ x by Def4
    .= FinUnion A.(y,x) by Def4;
end;
