def initialize(env=ENV)
    @env = env

    # note 'env' vs 'ENV'...
    @home     = env["GEM_HOME"] || ENV["GEM_HOME"] || Gem.default_dir

    if File::ALT_SEPARATOR then
      @home   = @home.gsub(File::ALT_SEPARATOR, File::SEPARATOR)
    end

    self.path = env["GEM_PATH"] || ENV["GEM_PATH"]
  end
#

Constructor. Takes a single argument which is to be treated like a
hashtable, or defaults to ENV, the system environment.