Base( G )
Base  returns a base  for the permutation group G.   If a  stabilizer
chain  for  G  is  already known,  Base  returns  the  base  for this
stabilizer chain.  Otherwise a stabilizer chain for the lexicographically
smallest  reduced  base  is  computed  and  its  base  is  returned  (see
Stabilizer Chains).
    gap> s4 := Group( (1,2,3,4), (1,2) );;
    gap> Base( s4 );
    [ 1, 2, 3 ]  
GAP 3.4.4