Patterns of conjunctive forks
  Three events in a probability space form a conjunctive fork if they satisfy specific constraints on conditional independence and covariances. Patterns of conjunctive forks within collections of events are characterized by means of systems of linear equations that have positive solutions. This characterization allows patterns of conjunctive forks to be recognized in polynomial time. Relations to previous work on causal betweenness and on patterns of conditional independence among random variables are discussed. 
Va\v{s}ek Chv\'atal, Franti\v{s}ek Mat\'u\v{s} and Yori Zw\'ol\v{s}
math.PR