proc differ(s1, s2) returns first_place
  var i := 1, len := min(length(s1),length(s2))
  do i <= len & s1[i] = s2[i] -> i++ od
  if i > len & length(s1) = length(s2) ->
      first_place := 0
  [] else ->
      first_place := i
  fi
end differ
