Another "proper class" are the operator precedence languages, AKA Floyd's languages (R. W. Floyd). They include the "visibly pushdown languages", which are real

Another "proper class" are the operator precedence languages, AKA Floyd's languages (R. W. Floyd). They include the "visibly pushdown languages", which are really just a rediscovery of the input-driven languages. Floyd's languages are the languages parsed using the shunting yard algorithm.