Thank you for your reply. I was trying to correct a typo when I encountered the problem. Yes, the problem is that I have a SI. It has been so long, I forgot about that.
I still think there is a small bug, however. Seeing the message 'problem 'Nabla.cc:444' would make anyone think there was an internal problem. APL had a standard message when that type of event occurred. GNU APL should display the standard error message when that type of user error occurred. This type of error can sometimes occur even by an experienced APL programmer (as I was). If the system displayed a message that looked more like user error, it would help. You could just look at the error message that the other APL's display (since I don't remember what the normal response was to that user error).
Cool system though!! I am look forward to your answers to my other email.
Thanks!