The examples seems to be nonsense. Fortunately the examples are not
what you really get.
(%i1) declare (F, nary);
(%o1) done
(%i2) F ([L]) := L;
(%o2) F([L]) := L
(%i3) xreduce (F, [a, b, c, d, e]);
(%o3) [[[[[("[", simp), a], b], c], d], e]
I actually got [a,b,c,d,e] instead of %o3. The line above reflects the
attempt to display a malformed expression, as far as I can see.