-- build a word tree (character occurrence tree) and walk it
pairs = {}
wordTree = {}
wordTreeP = wordTree
function p(m, s)
  if s ~= nil then
    for i = 1, s:len() do
      local c = s:sub(i, i)
      if c >= "a" and c <= "z" or c >= "A" and c <= "Z" then
        local n = wordTreeP[c]
        if n == nil then n = {} wordTreeP[c] = n end
        wordTreeP = n
      else
        wordTreeP = wordTree
      end
    end
  end
  local c, n = next(wordTreeP)
  return c
end