[rules-users] How to write rules with optional parts?

How do you do rules that are a sequence of events where some of them are optional? Example: Alert if DAVID enters a room after ALICE or after ALICE and BOB If it were a string, this is roughly equivalant to the regex /ALICE( BOB)? DAVID/ So: Alice then David => alert Alice then Bob then David => alert Alice then Charles then David => no alert I could write this as two rules but as the rules get more complex it could by hard to manage the resulting combatorial explosion. Here it is as two rules: Rule 1: $p1:person (name=="Alice") $p2:person (name=="David", this after[0,1] $p1) Rule 2: $p1:person (name=="Alice") $p2:person (name=="Bob", this after[0,1] $p1) $p3:person (name=="David", this after[0,1] $p2)