Catenation - The Other Useful Default
There is another logical default action for catenation, and that is to extend the leftmost dimension. This one is not as "absolutely unambiguous" as extension in the direction of increasing columns, because what you are adding (rows, planes, super-planes etc.) depends on the type of variable you are working with.
But this has many practical uses (adding records to a growing database etc.) that it is worth having it's own variation of the catenate symbol, and APL provides this as "cat bar" (the comma with a dash over it).
So we have gone "down the rabbit hole" of some basics in APL using the comma, and in doing so gone through some important concepts of data structure and default behaviour to see how APL is easy to read and write to do practical things. Maybe it is time to back out again and see how this applies to Metsim and process modelling and simulation.