Bernard Chazelle Thomas Ottmann Eljas Soisalon-Soininen Derick Wood The Complexity and Decidability of Separation. 119-127 1984 conf/icalp/1984 ICALP db/conf/icalp/icalp84.html#ChazelleOSW84 http://dx.doi.org/10.1007/3-540-13345-3_10