code: def referenced_node(id)
        nodes = document.xpath("//*[@ID='#{id}']")
  
        if nodes.size != 1
          raise SignatureError.new("Reference validation error: Invalid element references", "Expected 1 element with id #{id}, found #{nodes.size}")
        end
  
        nodes.first
      end
docs: Looks up node by id, checks that there's only a single node with a given id