When an error has occurred or when you interrupt GAP, usually by
hitting ctr-C, GAP enters a break loop, that is in most respects
like the main read eval print loop (see Main Loop). That is, you can
enter statements, GAP reads them, evaluates them, and prints the
result if any. However those evaluations happen within the context in
which the error occurred. So you can look at the arguments and local
variables of the functions that were active when the error happened and
even change them. The prompt is changed from gap to brk to
indicate that you are in a break loop.
There are two ways to leave a break loop.
The first is to quit the break loop and continue in the main loop. To do
this you enter quit; or hit the eof (end of file) character,
which is usually ctr-D. In this case control returns to the main
loop, and you can enter new statements.
The other way is to return from a break loop. To do this you enter
return; or return expr;. If the break loop was entered because you
interrupted GAP, then you can continue by entering return;. If the
break loop was entered due to an error, you usually have to return a
value to continue the computation. For example, if the break loop was
entered because a variable had no assigned value, you must return the
value that this variable should have to continue the computation.
GAP 3.4.4