integrate history-substring-search-down with existing widget

[blueray453]

My current widget associated with downkey looks like the following:

down-line-or-select() {
if [[ $RBUFFER == $'\n' ]]; then
zle down-line
else
zle menu-select -w
fi
}

zle -N down-line-or-select
bindkey $terminfo[kcud1] down-line-or-select # down arrow

How can I integrate history-substring-search-down with this?