// failures maps failed save attempts to the error that caused them (validation, conflict, etc.)
function(doc, req, failures) {
  var newDoc = doc || {_id: req.query.id || "example"};

  if(!(newDoc._id in failures)) {
    // Legacy code does this now. It knows nothing about the failures argument, but
    // just blindly attempts a save. Possibilities:
    // 1. Successful save. Normal code path.
    // 2. Conflict or invalid. Erlang has 2 choices.
    //    A. newDoc._id is not in failures; call me again with newDoc._id
    //       added to the failures object.
    //    B. newDoc._id is in failures. Erlang knows my code is naive or does
    //       not know the new API; Same 409 Conflict as before.
    return [newDoc, "Attempting a smart save: " + newDoc._id + "\n"];
  } else {
    var reason;
    if('validation' in failures[newDoc._id]) {
      reason = "Validation failure: " + JSON.stringify(failures[newDoc._id].validation);
    } else if ('conflict' in failures[newDoc._id]) {
      reason = "Document update conflict";
    } else {
      reason = 'Unknown reason';
    }
    return [null, "Sorry, this is not possible: " + reason + "\n"];
  }
}