There is a new global variable
fail
in GAP 4.
It is intended as a return value of a function for the case that it
could not perform its task.
For example, Inverse returns fail if it is called with a singular
matrix, and Position returns fail if the second argument is not
contained in the list given as first argument.
GAP 3 handled such situations by either signalling an error,
for example if it was asked for the inverse of a singular matrix,
or by (mis)using false as return value, as in the example Position.
Note that in the first example, in GAP 3 it was necessary to check
the invertibility of a matrix before one could safely ask for its
inverse, which meant that roughly the same work was done twice.
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GAP 4 manual