docs: function checkIfMatch(req, res, next) {
      var clientETags;
      var cur;
      var etag = res.etag || res.getHeader('etag') || '';
      var ifMatch;
      var matched = false;
  
      if ((ifMatch = req.headers['if-match'])) {
  
          clientETags = ifMatch.split(/\s*,\s*/);
  
          for (var i = 0; i < clientETags.length; i++) {
              cur = clientETags[i];
  
              // only strong comparison
  
              cur = cur.replace(/^W\//, '');
              cur = cur.replace(/^"(\w*)"$/, '$1');
  
              if (cur === '*' || cur === etag) {
                  matched = true;
                  break;
              }
          }
  
          if (!matched) {
              var err = new PreconditionFailedError(IF_MATCH_FAIL,
                  ifMatch,
                  etag);
              return (next(err));
          }
      }
  
      return (next());
  }