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 Th36:
  FinUnion A is associative
proof
  let x,y,z;
  thus FinUnion A.(FinUnion A.(x,y), z) = FinUnion A.(x \/ y, z) by Def4
    .= x \/ y \/ z by Def4
    .= x \/ (y \/ z) by XBOOLE_1:4
    .= FinUnion A.(x, y \/ z) by Def4
    .= FinUnion A.(x, FinUnion A.(y,z)) by Def4;
end;
