-- a, b are the last 2 words
pairs = {}
word = {}
nword = 0
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
        nword = nword+1
        word[nword] = c
      else
        if nword ~= 0 then
          newWord(table.concat(word))
          word = {}
          nword = 0
        end
      end
    end
  end
  local x = pairs[b]
  if x ~= nil then return " "..x else return nil end
end

function newWord(w)
  a = b
  b = w
  --print("newWord "..w)
  if a ~= nil then
    pairs[a] = b
  end
end

Began life as a copy of #1004710