# File lib/sup/util.rb, line 438
  def shared_prefix caseless=false, exclude=""
    return "" if empty?
    prefix = ""
    (0 ... first.length).each do |i|
      c = (caseless ? first.downcase : first)[i]
      break unless all? { |s| (caseless ? s.downcase : s)[i] == c }
      next if exclude[i] == c
      prefix += first[i].chr
    end
    prefix
  end