Case T-58/21: Judgment of the General Court of 20 December 2023 — Stadtwerke Hameln Weserbergland v Commission (Competition — Concentrations — German electricity and gas markets — Decision declaring a concentration compatible with the internal market — Obligation to state reasons — Concept of ‘single concentration’ — Right to effective judicial protection — Right to be heard — Definition of the market — Period of analysis — Assessment of the effects of the transaction on competition — Manifest errors of assessment — Undertakings — Duty of diligence)