GAP Manual: 45.9. IsBound

IsBound( rec.name )
IsBound( list[n] )

In the first form IsBound returns true if the record rec has a component with the name name, which must be an ident and false otherwise. rec must evaluate to a record, otherwise an error is signalled.

In the second form IsBound returns true if the list list has a element at the position n, and false otherwise. list must evaluate to a list, otherwise an error is signalled.

    gap> r := rec( a := 1, b := 2 );;
    gap> IsBound( r.a );
    true
    gap> IsBound( r.c );
    false
    gap> l := [  , 2, 3,  , 5,  , 7,  ,  ,   , 11 ];;
    gap> IsBound( l[7] );
    true
    gap> IsBound( l[4] );
    false
    gap> IsBound( l[101] );
    false 

Note that IsBound is special in that it does not evaluate its argument, otherwise it would always signal an error when it is supposed to return false.