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;
