The debug> prompt appears when an error occurs in interactive mode. Gamma halts execution of the program and produces the prompt. You can perform any action at the debug> prompt that you can perform at the top level, including modifying program source and setting variable values. The value of any variable can be queried by simply typing its name. The calling stack can be queried by using the stack function.

The stack function displays the execution stack, providing a list with the names of the nested functions executing when the error occurred. The outermost, or top level, function appears first (after progn). The function causing the error appears last on the list.

Once the debug> prompt appears, the program cannot be continued and must be re-started. If an error occurs again as a result of code executed within the debug level, another nested level of debug will appear. Each level adds to the current point on the execution stack. You can move up debug levels and return to the Gamma> prompt by pressing Ctrl - D at the debug> prompt.