 function vo = mtimes(a, vi)
%	MRI "forward projection" y=A*x and backprojection x = (A')*y

if a.is.empty
	error empty
end

Ain = a.Ain;
Aout = a.Aout;


if ~a.is.transpose
    
    vo = [Ain*vi;Aout*vi];
else
    nn = size(Ain);
    n1 = nn(1);
    %n2 = size(Aout,1);
    vo = Ain'*vi(1:n1) + Aout'*vi(n1+1:end);
    
end
