function mask = randmask(m,n)
mask = cell(1,n);
for i=1:n
  mm = find(round(rand(1,m)));
  % avoid triv. masks
  if isempty(mm)
    k = randperm(m);
    mm = k(1);
  end
  if length(mm)==m
    k = randperm(m);
    mm = setdiff(mm,k(1));
  end
  mask{i} = mm;
end
return