/* returns df, which will be canonical or NULL if any of the */
/* requirements are not met (if this case df is unchanged); that */
/* is, the input data must be as returned by decFloatToPacked, */
/* returns df, which will be canonical or NULL if any of the */
/* requirements are not met (if this case df is unchanged); that */
/* is, the input data must be as returned by decFloatToPacked, */