The prompt | ?- indicates that Prolog is waiting for a goal
to be typed in. For example, you can call built-in predicates like this:
| ?- write(hello).
hello
yes
| ?- X is 2+2.
X = 4 <RET>
| ?-
When Prolog prints a variable binding at the top level like X = 4 in this
example, it waits for you to either type a <RET>, which brings it back to
the top level, or else type a ;, which causes it to backtrack and look
for another solution. In this case, you would get
| ?- X is 2+2.
X = 4 ;
no
| ?-
because there is only one X for which the goal can be satisfied.
It is always possible to interrupt any Prolog process and return to the top-level Prolog prompt. To do this, type ^c. The system then displays the message
Prolog interruption (h for help)?
Type a (for abort) and press <RET>. The system then displays a message indicating that execution has been aborted, followed by the top-level Prolog prompt.
! Execution aborted
| ?-