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?