fold:=(f,a,v)->`if`(nops(v)=0,a,fold(f,f(a,op(1,v)),[op(2...,v)])):
insert:=(v,a,n)->`if`(n>nops(v),[op(v),a],subsop(n=(a,v[n]),v)):
perm:=s->fold((a,b)->map(u->seq(insert(u,b,k+1),k=0..nops(u)),a),[[]],s):
perm([$1..3]);
   [[3, 2, 1], [2, 3, 1], [2, 1, 3], [3, 1, 2], [1, 3, 2], [1, 2, 3]]