[TUHS] Posix ed weirdness

[Douglas McIlroy via TUHS]

The ed j command joins a sequence of lines into one line and sets dot
to the result. Thus
     2,4j
combines three lines and sets dot to 2. Naturally one expects
     2,2j
to make no visible change to the file, but to set dot to 2.

Indeed, that's what v7 did. But Posix decrees that j "does nothing" in
this case and leaves dot at the value it had before the command.

Does anyone know why the Posix committee chose to break both the
original behavior and  Kernighan's law: " 'Do nothing' gracefully"?

Doug