def install dep_or_name, version = Gem::Requirement.default
    if String === dep_or_name then
      find_spec_by_name_and_version dep_or_name, version, @prerelease
    else
      dep_or_name.prerelease = @prerelease
      @specs_and_sources = [find_gems_with_sources(dep_or_name).last]
    end

    @installed_gems = []

    gather_dependencies

    last = @gems_to_install.size - 1
    @gems_to_install.each_with_index do |spec, index|
      next if Gem::Specification.include?(spec) and index != last

      # TODO: make this sorta_verbose so other users can benefit from it
      say "Installing gem #{spec.full_name}" if Gem.configuration.really_verbose

      _, source_uri = @specs_and_sources.assoc spec
      begin
        local_gem_path = Gem::RemoteFetcher.fetcher.download spec, source_uri,
                                                             @cache_dir
      rescue Gem::RemoteFetcher::FetchError
        next if @force
        raise
      end

      inst = Gem::Installer.new local_gem_path,
                                :bin_dir             => @bin_dir,
                                :development         => @development,
                                :env_shebang         => @env_shebang,
                                :force               => @force,
                                :format_executable   => @format_executable,
                                :ignore_dependencies => @ignore_dependencies,
                                :install_dir         => @install_dir,
                                :security_policy     => @security_policy,
                                :user_install        => @user_install,
                                :wrappers            => @wrappers

      spec = inst.install

      @installed_gems << spec
    end

    @installed_gems
  end
#
Installs the gem +dep_or_name+ and all its dependencies.  Returns an Array
of installed gem specifications.

If the +:prerelease+ option is set and there is a prerelease for
+dep_or_name+ the prerelease version will be installed.

Unless explicitly specified as a prerelease dependency, prerelease gems
that +dep_or_name+ depend on will not be installed.

If c-1.a depends on b-1 and a-1.a and there is a gem b-1.a available then
c-1.a, b-1 and a-1.a will be installed.  b-1.a will need to be installed
separately.