When a single legislator wishes to leave her seat, Article 101(3)(b) requires her to submit a written resignation to the presiding officer, who must be satisfied that it is voluntary and genuine. That is the constitutional treatment of one legislator unmaking one mandate: a formal act, a named officer, a test of authenticity. Paragraph 4 permits the collective reassignment of two-thirds of a legislature party’s mandates and attaches none of these safeguards. The same presiding officer who must probe the authenticity of one resignation has no duty to probe anything when seven MPs arrive with a joint letter of merger.