gap> f := function()
>     return 1 - "abc";
> end;
function(  ) ... end
gap> f();
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `AdditiveInverseMutable' on 1 arguments
Stack trace:
*[1] AdditiveInverseMutable( elm )
   @ GAPROOT/lib/arith.gi:LINE
 [2] AdditiveInverseImmutable( elm )
   @ GAPROOT/lib/arith.gi:LINE
 [3] 1 - "abc"
   @ *stdin*:3
<function "f">( <arguments> )
 called from read-eval loop at *stdin*:5
type 'quit;' to quit to outer loop
brk> QUIT;
