def index_signature
      require 'digest'

      Digest::SHA256.new.hexdigest(@gems.keys.sort.join(',')).to_s
    end
def index_signature
      require 'digest'

      Digest::SHA256.new.hexdigest(@gems.keys.sort.join(',')).to_s
    end
The signature for the source index.  Changes in the signature indicate a
 change in the index.