function ufilt=plfilt(u,n)
%              ufilt=plfilt(u,[n])
%           loads and uses pl33 filter
%           with no arguments, it returns a series the same length 
%           as the original, with nans at either end.
%           If n is provided, it resamples by n, starting at point 34.
load pl33
ufilt=conv(u,pl33);
ufilt=ufilt(34:length(u)+33);
ufilt(1:33)=ones(33,1)*nan;
ufilt(length(ufilt)-32:length(ufilt))=ufilt(1:33);
if nargin>1
    ufilt=ufilt(34:n:length(ufilt)-33);
end

