alan :-
	intape(Is),
	( Is = [I|IRest] ->
		perform(q0, [], Ls, [I], Rs, IRest, _)
	;
		perform(q0, [], Ls, [b], Rs, [], _)
	),
	reverse(Ls, Ls1),
	append(Ls1, Rs, Band),
	maplist(write, Band).

perform(qf, Ls, Ls, Rs, Rs, I, I) :- !.
perform(Q0, Ls0, Ls, [R|Rs0], Rs, I0, I) :-
	rule(Q0, R, Q1, Sym, Action),
	!, % let's hope the TM is deterministic
	action(Action, Ls0, Ls1, [Sym|Rs0], Rs1, I0, I1),
	perform(Q1, Ls1, Ls, Rs1, Rs, I1, I).

action(left, Ls0, Ls, Rs0, Rs, I, I) :-
	( Ls0 = [] ->
		Ls = [],
		Rs = [b|Rs0]
	;
		Ls0 = [L|Ls],
		Rs = [L|Rs0]
	).
action(stay, Ls, Ls, Rs, Rs, I, I).
action(right, Ls0, [Sym|Ls0], [Sym|Rest], Rs, I0, I) :-
	( Rest = [] ->
		( I0 = [FI|I0Rest] ->
			Rs = [FI],
			I = I0Rest
		;
			Rs = [b],
			I = []
		)
	;
		Rs = Rest
	).