STATIC void
S_initialize_invlist_guts(pTHX_ SV* invlist, const Size_t initial_size)
{
    PERL_ARGS_ASSERT_INITIALIZE_INVLIST_GUTS;

    /* First 1 is in case the zero element isn't in the list; second 1 is for
     * trailing NUL */
    SvGROW(invlist, TO_INTERNAL_SIZE(initial_size + 1) + 1);
    invlist_set_len(invlist, 0, 0);

    /* Force iterinit() to be used to get iteration to work */
    invlist_iterfinish(invlist);

    *get_invlist_previous_index_addr(invlist) = 0;
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