# File lib/abbrev.rb, line 44
  def abbrev(words, pattern = nil)
    table = {}
    seen = Hash.new(0)

    if pattern.is_a?(String)
      pattern = /^#{Regexp.quote(pattern)}/     # regard as a prefix
    end

    words.each do |word|
      next if (abbrev = word).empty?
      while (len = abbrev.rindex(/[\w\W]\z/)) > 0
        abbrev = word[0,len]

        next if pattern && pattern !~ abbrev

        case seen[abbrev] += 1
        when 1
          table[abbrev] = word
        when 2
          table.delete(abbrev)
        else
          break
        end
      end
    end

    words.each do |word|
      next if pattern && pattern !~ word

      table[word] = word
    end

    table
  end