code: public static Vec categoricalToStringVec(Vec src) {
      if( !src.isCategorical() )
        throw new H2OIllegalValueException("Can not convert a non-categorical column"
            + " using categoricalToStringVec().",src);
      return new Categorical2StrChkTask(src.domain()).doAll(Vec.T_STR,src).outputFrame().anyVec();
    }
docs: Create a new {@link Vec} of string values from a categorical {@link Vec}.
  
  Transformation is done by a {@link Categorical2StrChkTask} which provides a mapping
  between values - without copying the underlying data.
  
  @param src a categorical {@link Vec}
  @return a string {@link Vec}