Socialwg/Social API/Identity to API endpoints

This is rough pseudocode for discovering the "friends" API endpoint for a given URI.

let f = "http://example.com/social-api/friends"
let u = the identity URI
let p = protocol of u

if p == "http" or p == "https"

  let hr = send a HEAD request to server for u
  if hr includes a Link: header for the f relation
    return the value of that header
  else
    let gr = send a GET request to the server for u
    if gr has content type "text/html"
      let ht = HTML.parse gr.body
      if ht includes a <link> element with rel = f
        return href of that <link> element
      else if ht includes an <a> element with rel = f
        return href of that <a> element
      else
        return null        
    else if gr has content type "application/activity+json"
      let ao = JSON.parse gr.body
      return link-relation f from ao
    else if gr has content type "?"
      // Other discovery types here...
    else
      return null

 else if p == "acct"

   do Webfinger discovery of link relation f for identity u