Middle Mouse Button to mark/unmark a stage green, Right Mouse Button to mark/unmark a stage red
Ctrl + Middle Mouse Button to disconnect a path, this will disconnect the stage connected to the one clicked,
for example if you have BOB→WF→JRB→BBH→HMC→LLL and you hold Ctrl and click with the middle mouse button on JRB
you'll get two paths: BBH→HMC→LLL and BOB→WF→JRB
Ctrl + Left clicking a stage that is not in a path will put it in its own path, looping back to itself.