NetLogo/NetLogo — pull request #590 by qiemem (2014-04-11 19:06:38+00:00)
Title: Add variable watchers

See [these tests](https://github.com/NetLogo/NetLogo/blob/4a6ad21ae168181d9a05d14f0ebd826ff7e1c173/src/test/org/nlogo/headless/TestAgentVariableObservers.scala#L27) for examples of use.