docs: public IsNullValue downgradeOnControlSplit() {
          IsNullValue value = this;
  
          if (NCP_EXTRA_BRANCH) {
              // Experimental: track two distinct kinds of "null on complex path"
              // values.
              if (value.isNullOnSomePath()) {
                  value = nullOnComplexPathValue();
              } else if (value.equals(nullOnComplexPathValue())) {
                  value = nullOnComplexPathValue3();
              }
  
          } else {
              // Downgrade "null on simple path" values to
              // "null on complex path".
              if (value.isNullOnSomePath()) {
                  value = nullOnComplexPathValue();
              }
          }
          return value;
      }