For a derivation, after specifying inference rule, prompt for correct number of inputs and feeds. (Currently input expressions are on a separate page because number of inputs is a server-side lookup.)
If inference rule and input and feed are specified, guess output expression dynamically. (Currently output expressions have to be manually specified.)
See https://flask.palletsprojects.com/en/2.3.x/patterns/javascript/
Two separate possible use cases: