function u=construct(s,sizes_in)

	nin1=nargin(s.a1);
	nin2=nargin(s.a2);

	u1=[]; u2=[];
	try
		u1=construct(s.a1,sizes_in(1:nin1));
		u2=construct(s.a2,sizes_in(nin1+(1:nin2)));

		u.sizes_out = [u1.sizes_out,u2.sizes_out];

		nout1=length(u1.sizes_out);
		nout2=length(u2.sizes_out);

		ix1 = 1:nin1;  ix2 = nin1+(1:nin2); 
		ox1 = 1:nout1; ox2 = nout1+(1:nout2);

		nout = length(u.sizes_out);
		if all([nin1,nin2,nout1,nout2]==[1,1,1,1]),
			u.process = @proc11;
		else
			u.process = @proc;
		end
	catch ex
		if ~isempty(u2), u2.dispose(); end
		if ~isempty(u1), u1.dispose(); end
		rethrow(ex);
	end

	u.dispose = @dispose;
	u.starting= @starting;
	u.stopping= @starting;
	u.dispose = @dispose;
	u.get_state = @get_state;
	u.set_state = @set_state;
	u.viewables = [u1.viewables;u2.viewables];

	function [y1,y2]=proc11(x1,x2)
		y1=u1.process(x1);
		y2=u2.process(x2);
	end

	function varargout=proc(varargin)
		varargout=cell(1,nout);
		[varargout{ox1}]=u1.process(varargin{ix1});
		[varargout{ox2}]=u2.process(varargin{ix2});
	end

	function dispose, u1.dispose(); u2.dispose(); end
	function starting, u1.starting(); u2.starting(); end
	function stopping, u1.stopping(); u2.stopping(); end

	function s=get_state, s = {u1.get_state(),u2.get_state()}; end
	function set_state(s), u1.set_state(s{1}); u2.set_state(s{2}); end
end

