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 Th34:
  FinUnion A is idempotent
proof
  let x;
  thus FinUnion A.(x,x) = x \/ x by Def4
    .= x;
end;
