!TITLE Program Execution And Interruption
!KEY
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 hitting the interrupt key on your terminal (normally marked
ESC).  In response to the prompt

            Int:

you can type either "a" or "d" (or "A" or "D").  The "a"  response  will  force
Prolog  to  abort  back  to top level, whereas "d" will switch on debugging and
continue the execution.  If you are having trouble and not  getting  any  reply
from  Prolog  (perhaps  your program is in a loop), then typing ESC followed by
"a" (and then <return>) should get you back to Prolog top-level.