def traverse trail = [], &block
    trail = trail + [self]
    runtime_dependencies.each do |dep|
      dep.to_specs.each do |dep_spec|
        block[self, dep, dep_spec, trail + [dep_spec]]
        dep_spec.traverse(trail, &block) unless
          trail.map(&:name).include? dep_spec.name
      end
    end
  end
#
Recursively walk dependencies of this spec, executing the +block+ for each
hop.