def to_id(fei)

      a = [ Ruote.to_storage_id(fei) ]

      a.unshift(@store_name) if @store_name

      a.unshift('wi')

      a.join('!')
    end
def to_id(fei)

      a = [ Ruote.to_storage_id(fei) ]

      a.unshift(@store_name) if @store_name

      a.unshift('wi')

      a.join('!')
    end
Computes the id for the document representing the document in the storage.