Normally the entries in a mutable list are not printed, to prevent infinite loops in the printing routines. To print them out, use peek.
i1 : s = new MutableList from {a,b,c};
i2 : s#2 = 1234;
i3 : s o3 = MutableList{...} o3 : MutableList
i4 : peek s o4 = MutableList{a,b,1234} o4 : Net
See also BasicList.
