code: public String toCanonicalSignatureString(int height) {
          String canonicalSignature = null;
          for (int i = 0; i < getVertexCount(); i++) {
              String signatureForI = signatureStringForVertex(i, height);
              if (canonicalSignature == null || canonicalSignature.compareTo(signatureForI) < 0) {
                  canonicalSignature = signatureForI;
              }
          }
          return canonicalSignature;
      }
docs: Make a canonical signature string of a given height.
  
  @param height the maximum height to make signatures
  @return the canonical signature string