```\nbytes32 urlHash = keccak256(bytes(\_url));\n\n// make sure this url and also this owner was not registered before.\n// solium-disable-next-line\nrequire(!urlIndex[urlHash].used && signerIndex[\_signer].stage == Stages.NotInUse,\n    "a node with the same url or signer is already registered");\n```\n