To the extent that the liturgy is not a static, fossilized entity but an organic one in need of tending, I've wondered about the process in which propers were created and assigned. When discussions are posted regarding whether to choose an antiphon from the GR versus an alius cantus aptus, it appears to me that the main consideration is schola ability. After reading a fair amount of McKinnon, I wonder whether all propers are created equal. Even if McKinnon's idea that parts of the liturgical cycle show greater planning is incorrect, it certainly seems to be the case that some chants are more stable than others. One need only look at Dobszay's index in "The Genius of the Roman Rite". In other words, the Easter gradual Haec Dies seems to me to be part of a well planned set, very ancient (see Peter Jeffery's "The Lost Chant Tradition") but also stable over time and geography (see Dobszay). Other chants, particularly in "ordinary time" appear to me at least less stable geographically and temporally (e.g. the Alleluia Paratum Cor Meum), and perhaps less ancient or perhaps indicative of less planning. I'm curious as to whether these sorts of concerns fit into programming decisions, and if so, which propers have been considered more dispensible than others?