items.copy_to(new_str, bytelen, index_from, 0)
o.items.copy_to(new_str, o.bytelen, o.index_from, bytelen)
return new FlatString.full(new_str, 0, new_bytelen - 1, new_bytelen, newlen)
items.copy_to(new_str, bytelen, index_from, 0)
o.items.copy_to(new_str, o.bytelen, o.index_from, bytelen)
return new FlatString.full(new_str, 0, new_bytelen - 1, new_bytelen, newlen)