Type Alias RehearseOptions<State, Command, Flags>

RehearseOptions: [Flags] extends [never]
    ? Readonly<{ from?: Step<State, Command> }>
    :
        | Readonly<{ flags?: never; from: Step<State, Command> }>
        | Readonly<{ flags: Flags; from?: never }>

Where a rehearsal starts: from, or the program's init, from flags for a program with them, which then needs one of the two.