Debugging with GT.M

This information is from the GT.M Programmer's Manual. I'll just summarize a few points.

 $ECODE
   This is a list of past errors, separated by commas. 
   Subsequent errors are added to the end of the list.
   example:
     GTM>w $ECODE
     ,M6,Z150373850, 
   Errors beginning with: "M" are standard errors
                          "U" are user errors
                          "Z" are GT.M errors

 $ZSTATUS
   This is detail about the most recent error.
   example:
     GTM>w $ZSTATUS
     150373850,SubName+3^MyFunct,%GTM-E-UNDEF, Undefined local variable: Jxack