:: (a, b)

Some Tardises never observe the initial state of either state stream, so it is convenient to simply hand dummy values to such Tardises.
noState = (undefined, undefined)
Indicates that the request does not match the current handler.
An empty Graph.
the zero arrow, alias for zeroArrow
read the complete state, ignore arrow input definition: getState = accessState (\ s x -> s)
Obtain the current value of the state.
Obtain the current value of the state.
Raise an error.