Flat( list )
Flat returns  the list of all  elements that are  contained in the list
list or its sublists.   That  is, Flat first makes  a new empty  list
new.  Then it loops over the elements elm of list.  If elm is not
a list it is added to new, otherwise Flat appends Flat( elm  ) to
new.
    gap> Flat( [ 1, [ 2, 3 ], [ [ 1, 2 ], 3 ] ] );
    [ 1, 2, 3, 1, 2, 3 ]
    gap> Flat( [ ] );
    [  ] 
GAP 3.4.4