How to pass an XPath expression as parameter?

declare function local:insert(
  $doc as node(),
  $at-location as xpath(),  (: I know, there is no 'xpath' type, this is
just for demo :)
  $what as node()*
) as node() {
  copy $c := $doc
  modify insert node $what into $at-location
  return $c
};

let $xml := <xml><body><p>This is a paragraph.</p></body></xml>
let $nd := <this><is><a>Test</a></is></this>
return local:insert($xml, xpath("/body"), $nd)

declare function local:insert($doc, $at-location, $what) {
  let $update := ``[
  declare variable $doc external;
  declare variable $what external;
  declare variable $at-location external;
copy $c := $doc
modify insert node $what into `{$at-location}`
return $c
]``
  return xquery:eval($update, map{
    'doc':$doc,
    'at-location':$at-location,
    '$what':$what
  })
};

let $xml := <xml><body><p>This is a paragraph.</p></body></xml>
let $node := <this><is><a>Test</a></is></this>
return local:insert($xml, "$c/body", $node)

declare function local:add-node(
  $input as node()*,
  $what as node()*
) as node() {
  copy $c := $input
  modify insert node $what into $c
  return $c
};
declare function local:add-node(
  $input as node()*,
  $what as node()*,
  $at-location as xs:string
) as node() {
  copy $c := $input
  modify
    let $xpath := xquery:eval($at-location, map{"":$c})
    return
      insert node $what into $c/$xpath
  return $c
};

  declare function local:add-nodes1(
    $input as node(),
    $nodes as node()*,
    $path as xs:string
  ) as node() {
    $input update {
      let $target := xquery:eval($path, map { '': . })
      return insert nodes $nodes into $target
    }
  };

  declare function local:add-nodes2(
    $input as node(),
    $nodes as node()*,
    $path as xs:string
  ) as node() {
    $input update {
      let $steps := tokenize($path, '/')
      let $target := fold-left($steps, ., function($node, $name) {
        $node/*[name() = $name]
      })
      return insert nodes $nodes into $target
    }
  };