def spec_predecessors
    result = Hash.new { |h,k| h[k] = [] }

    specs = @specs.sort.reverse

    specs.each do |spec|
      specs.each do |other|
        next if spec == other

        other.dependencies.each do |dep|
          if spec.satisfies_requirement? dep then
            result[spec] << other
          end
        end
      end
    end

    result
  end
#
Return a hash of predecessors.  <tt>result[spec]</tt> is an Array of
gemspecs that have a dependency satisfied by the named gemspec.