fo:bidi-override - is it necessary?

The description of fo:bidi-override in the XSL spec says, "The
fo:bidi-override formatting object is used when the Unicode-bidi
algorithm fails. It forces a string of text to be written in a
specific direction."

Unicode already defines several characters whose sole purpose is to
control (and possibly override) the formatting behavior: LRE, RLE,
LRO, RLO, LRM, RLM and PDF.

What functionality, if any, does fo:bidi-override offer that isn't
already provided?

-Steve


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