Is it permissible to shorten the unconfirmed Sunnah prayer to a single rak'ah?

Unconfirmed Sunnah prayers, such as the two rak'ahs after ablution and the pre-Maghrib Sunnah prayer, are not shortened; rather, they are prayed as two rak'ahs, because the Sharia has specified them as two rak'ahs. The prayers that are shortened are the four-rak'ah obligatory prayers. Performing a supererogatory prayer with a single rak'ah is prohibited by many jurists due to the Prophet's (peace be upon him) saying: "The night and day prayers are two by two," and no prayer of a single rak'ah is known in the Sharia other than Witr. However, others view its permissibility by analogy with Witr, and based on the hadith: "Prayer is the best subject [of good deeds]; whoever wishes can do little, and whoever wishes can do much." But the preponderant opinion is that performing a supererogatory prayer with a single rak'ah is not valid, because the hadiths that permit it are weak or do not relate to the Sunnah prayers for which the Sharia has specified the number of their rak'ahs.