component arguments starting with _ eg _foo are private

[amitu]

-- component foo:
caption title:             ;; public
boolean $_open: false      ;; private

-- ftd.text: $foo.title
$on-click$: $ftd.toggle($a = $foo._open)
color if { foo._open }: red

-- end: foo

[amitu]

This is superseded by https://github.com/orgs/fastn-stack/discussions/1816.