let rec findTransition env = function
    |(endstate, edge)::rest ->
        match edge with
        |Ass(var, aexpr) ->if (aexpr.Valid env) then (endstate, Map.add var (aexpr.Eval env) env) else findTransition env rest
        |Skip -> (endstate, env)
        |Conditional(bexpr) -> if bexpr.Eval env then (endstate, env) else findTransition env rest
    | [] -> failwith("Unable to fire any transition")

let rec execute s g env = Map.find s g |> findTransition env |> fun (s':string,env') -> if s'.Contains "◀" then "terminated",env' else execute s g env'