I think it would be a mistake to bake too many restrictions into the
general framework. There is a certain mental model that prevails in
LCA, but we don't want BONSAI to accept these restrictions at the
beginning unless they are absolutely necessary, and BONSAI is not just
for LCA (e.g. should also be useful for MFA). For now it might be
worth skipping the determining flow completely, as it doesn't seem
necessary for the hackathon.
Determining flows are not always outputs, treatment of waste by
landfill has waste as a determining flow input.