code: int findPrimary(long p) {
          // Requirement: p must occur as a root primary.
          assert((p & 0xff) == 0);  // at most a 3-byte primary
          int index = findP(p);
          // If p is in a range, then we just assume that p is an actual primary in this range.
          // (Too cumbersome/expensive to check.)
          // Otherwise, it must be an exact match.
          assert(isEndOfPrimaryRange(elements[index + 1]) || p == (elements[index] & 0xffffff00L));
          return index;
      }
docs: Finds the index of the input primary.
  p must occur as a root primary, and must not be 0.