Execution  of  a  program  is started by giving the interpreter a directive
which contains a call to one of the program's procedures.

Only  when  execution  of  one  directive  is complete does the interpreter
become  ready for another directive.  However, one may interrupt the normal
execution  of a directive by typing <delete> This <delete> interruption has
the  effect  of  terminating the execution of the command.  The system will
then respond with a prompt.

Execution  may  also  be terminated if the program runs out of stack space.
When  this  occurs,  the  user  may  save the current program and reexecute
Prolog  with  more stack space by using the -s option in the shell command.
Warning:   make sure that the reason the program ran out of stack space was
not due to an error in the program before increasing the size of Prolog.

NOTE:  This is the UNSW Prolog interpretter stack, NOT the AmigaDOS stack.