@ARTICLE{DasPauSal_j1993_a, author = {J{\"u}rgen~Dassow and Gheorghe~P\u{a}un and Arto~Salomaa}, title = {On the union of {OL} languages}, journal = {Inform. Process. Letters}, year = 1993, volume = 47, number = {}, pages = {59--63} }