@ARTICLE{CsuDasKelPau_j1994_b, author = {Erzs\'ebet~Csuhaj-Varj\'u and J{\"u}rgen~Dassow and Jozef~Kelemen and Gheorghe~P\u{a}un}, title = {Stratified grammar systems}, journal = caai, year = 1994, volume = 13, number = {}, pages = {409--422} }