Les Koehler taught me this trick that I have put to wide use since the mid-90s. Yes, adding a row to a stem array is a fairly simple process:
zz = log.0 + 1 log.zz = msgtext log.0 = zz
That is an example of using "a bigger hammer" even though many REXX programmers will look at it and exclaim "There's nothing wrong with that!" Indeed, there's nothing wrong with it... except that it's slow. If you're doing it several thousand times, you'll probably want something a little quicker. In fact, after you've found that 'something quicker', you may well decide to always use the quicker method. Those who don't follow REXXpertise will cock their heads to one side as if to ask "What in the world are you doing?" You'll get a chance then to explain the process to them ;-)
parse value log.0+1 msgtext with, zz log.zz 1 log.0 .
Here we first construct a value-string composed of "log.0 + 1" and "msgtext". This is parsed as "zz" and "log.zz". Since "zz" is set first (from the value of "log.0+1"), "log.zz" now points at the next available slot. The location pointer is reset to "1" and the parse continues, loading "log.0" with the incremented value. The remainder of the line is discarded. Once you understand the protocol, it makes perfect sense.