This paper describes a guiding principle for designing functional single-electron tunneling (SET) circuits - that is a way to elicit the potential functions of a given SET circuit by using as a guiding tool the SET circuit stability diagram. A stability diagram is a map that depicts the stable regions of a SET circuit based on the circuit's variable coordinates. By scrutinizing the diagram, we can infer all the potential functions that can be obtained from a circuit configuration. As an example, we take up a well-known SET-inverter circuit and uncover its latent functions by studying the circuit configuration, based on its stability diagram. We can produce various functions, e.g., step-inverter, Schmidt-trigger, memory cell, literal, and stochastic-neuron functions. The last function makes good use of the inherent stochastic nature of single-electron tunneling, and can be applied to Boltzmann-machine neural network systems.