theorem
  NOT NOT F = F
  proof
    thus NOT NOT F = id(X\dom NOT F) by Th37
    .= id (X\dom id(X\dom F)) by Th37
    .= id (X\(X\dom F)) by RELAT_1:45
    .= id (X/\dom F) by XBOOLE_1:48
    .= id dom F by XBOOLE_1:28
    .= F by Th45;
  end;
