Enumerator::Lazy#_enumerable_with_index

static VALUE
enumerator_with_index(int argc, VALUE *argv, VALUE obj)
{
    VALUE memo;

    rb_check_arity(argc, 0, 1);
    RETURN_SIZED_ENUMERATOR(obj, argc, argv, enumerator_enum_size);
    memo = (!argc || NIL_P(memo = argv[0])) ? INT2FIX(0) : rb_to_int(memo);
    return enumerator_block_call(obj, enumerator_with_index_i, (VALUE)MEMO_NEW(memo, 0, 0));
}