add comment to <List<ExistentialPredicates>>::principal
add comment to <List<ExistentialPredicates>>::principal

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust