  The lambda calculus is not upward confluent, one of counterexamples known
thanks to Plotkin. This paper investigates upward confluence in the interaction
calculus. Can an interaction system have this property? We positively answer
this question and also provide a necessary and sufficient condition for
stronger one-step upward confluence which happens to be very restrictive.
However, the provided condition is not necessary for upward confluence as we
prove that the interaction system of the linear lambda calculus is upward
confluent.
