From: Rabbi Moshe Sofer
To: Rabbi Sender Meizlish
Rabbi Sender questions the validity of an eruv constructed with a jointed beam on hinges that could be extended to form a tzuras hapesach but was unrecognizable as a doorway when folded upright. The Chasam Sofer agrees the arrangement is problematic due to lack of recognizability (hekera), but offers a strained justification based on treating the alleyway as a courtyard.
Source: Shu"t Chasam Sofer OC §90