function u=construct(s,sizes_in)
	nin=nargin(s.base);
	nout=nargout(s.base);

	running=0;
	current_arrow = s.base;
	current_unit  = construct(current_arrow,sizes_in(1:nin));
	current_proc  = current_unit.process;

	u=mkunit(s);
	u.sizes_out = current_unit.sizes_out;
	u.starting = @starting;
	u.stopping = @stopping;
	u.dispose  = @dispose;
	u.get_state = @get_state;
	u.set_state = @set_state;

	if     nin==1 && nout==1, u.process=@proc_11;
	elseif nin==1 && nout==0, u.process=@proc_10; 
	elseif nin==0 && nout==1, u.process=@proc_01; 
	else,  u.process=@proc_nn; end

	function starting, current_unit.starting(); running=1; end
	function stopping, current_unit.stopping(); running=0; end
	function dispose,  current_unit.dispose(); end

	function s=get_state,  s={current_arrow, current_unit.get_state()}; end
	function set_state(s), replace_running_unit(s{1},s{2}); end
		
	function y1=proc_11(x1,x), 
		if ~isempty(x), replace_running_unit(x{1}); end
		y1=current_proc(x1); 
	end

	function x=proc_10(x1,x), 
		if ~isempty(x), replace_running_unit(x{1}); end
		current_proc(x1); 
	end
	function y1=proc_01(x),
		if ~isempty(x), replace_running_unit(x{1}); end
		y1=current_proc(); 
	end

	function varargout=proc_nn(varargin)
		x=varargin{end};
		if ~isempty(x), replace_running_unit(x{1}); end
		[varargout{1:nout}]=current_proc(varargin{1:nin});
	end

	function replace_running_unit(a,state)
		if running 
			current_unit.stopping(); 
			current_unit.dispose(); 
		end
		current_arrow=a;
		current_unit=construct(a,sizes_in(1:nin));
		current_proc=current_unit.process;
		if nargin>1, current_unit.set_state(state); end
		if running, current_unit.starting(); end
	end
end


