cdm/src/main/java/ucar/nc2/dataset/CoordinateSystem.java
private CoordinateAxis lesserRank( CoordinateAxis a1, CoordinateAxis a2) {
    if (a1 == null) return a2;
    return (a1.getRank() <= a2.getRank()) ? a1 : a2;
  }
private CoordinateAxis lesserRank( CoordinateAxis a1, CoordinateAxis a2) {
    if (a1 == null) return a2;
    return (a1.getRank() <= a2.getRank()) ? a1 : a2;
  }
prefer smaller ranks, in case more than one