User blog comment:SuperSpruce/The T Array Function Part 4: Double Comma T Function (DCT)/@comment-5150073-20190606152814/@comment-27516045-20190606202919

I don't know the set-theoretic meaning that well, but I do konw that \(\psi(\Omega_2\) and \(\psi(\psi_1(\Omega_2))\) are often used interchangeably and, in most commonly-used OCFs, are the same and both valid.