A filter primitive that translates between a discriminated union
    and its corresponding external representation. The xdr_union
    routine first translates the discriminant of the union located at
    dscmp. This discriminant is always of type enum_t.

    Next, the routine translates the union data located at unp. To
    translate the union data the xdr_union routine first searches
    the structure pointed to by the choices argument for the union
    discriminant passed in the dscmp argument. If a match is found,
    the xdr_union routine calls proc to translate the union data.

    The end of the xdr_discrim structure array must contain an entry
    with the value NULL for proc. If the xdr_union routine reaches
    this entry before finding a match, the routine calls the default
    procedure (if it is not NULL).