MODULE delay(input)
    -- Model of the delay component
    VAR
        x : boolean;
    ASSIGN
        init(x) := FALSE;
        next(x) := input;
    DEFINE
        out := x;
 
MODULE relay(input)
    -- Model of the relay
    DEFINE
        out := input; 
        
MODULE inverter(input)
    -- Model of the inverter
    DEFINE
        out := !input; 


MODULE main
    -- Composition of delay and inverter
    VAR
        del : delay(inv.out);
        inv : inverter(del.out);
        
INVARSPEC inv.out = !del.out;