Trigger.SourceSourceSource for signaling a trigger.
t is the type of sources of triggers. A source can be used to signal the associated trigger.
same l r determines whether l and r are the one and the same trigger.
signal t signals t, running the callback registered with on_signal t ~f k if it exists. If t is already signalled then signal t does nothing.
is_signalled t is true if t has been signalled and false if it is unsignalled.