function next_character(candidates,w,s,i) =
if (i == #w) then candidates
else
  let letter    = w[i];
      next_l    = s->{c + i: c in candidates};
      candidates = {c in candidates; n in next_l | n == letter};
  in next_char(candidates, w, s, i+1);

function string_search(w, s) = next_character([0:#s - #w + 1],w,s,0);