code: function findFixedMatchFromHead(el, head) {
          var relativeOp, cur = head;
          do {
              if (!singleMatch(el, cur)) {
                  return null;
              }
              cur = cur.prev;
              if (!cur) {
                  return true;
              }
              relativeOp = relativeExpr[cur.nextCombinator];
              el = dir(el, relativeOp.dir);
          } while (el && relativeOp.immediate);
          if (!el) {
              return null;
          }
          return {
              el: el,
              match: cur
          };
      }
docs: find fixed part, fixed with seeds find fixed part, fixed with seeds