function D = selectx( A, I )
    % D = SELECTCLASSES(A, C)
    %
    % Return a new PRTools dataset containing only instances 
    % given in vector I.
    
    % We need to have the number of features (k) and the feature size
    % needs to be copied from the original to the new data set.
    [ m k ] = size(A);
    D = A( I, [1:k]);
    D.featsize = A.featsize;