function Xmin = ConvMin(X,t0,t1,F);
% function Xmin = ConvMin(X,t0,t1,F);
% convolution min
% Xmin is the running min
% from relative times t0 to t1, e.g. -12 to 12
% if less than F (fraction of 1) of the values in the 
% interval are available then the average is set to nan.

if nargin < 4,
   F = 2/3;
end

Xmin = X*nan;
N = zeros(size(X));

for t = t0:t1,
   Xt = ListGet(X,t);
   f = find((Xt < Xmin) | isnan(Xmin));
   Xmin(f) = Xt(f);
   N = N+~isnan(Xt);
end

f = find(N < F*(t1-t0+1));
Xmin(f) = nan;