On 2/11/15 6:10 PM, Andrew Sullivan wrote:
I like this proposed change better.
So, putting it all together and tweaking a few words here and there...
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5.2.5. Directionality Rule
The directionality rule of a profile specifies how to treat strings
containing what are often called "right-to-left" (RTL) characters
(see Unicode Standard Annex #9 [UAX9]). RTL characters come from
scripts that are normally written from right to left and are
considered by Unicode to, themselves, have right to left
directionality. Strings containing RTL characters often also contain
"left-to-right" (LTR) characters, such as numerals, as well as
characters without directional properties. Consequently, such
strings are known as "bidirectional strings". Using bidirectional
strings in different layout contexts can yield display results that,
while predictable to those who understand the display rules, are
counter-intuitive to casual users. Therefore some profiles might
wish to restrict the use of bidirectional strings by specifying a
directionality rule.
The PRECIS framework does not directly address how to deal with
bidirectional strings, since there is currently no widely accepted
and implemented solution for the safe display of arbitrary
bidirectional strings beyond the Unicode bidirectional algorithm
[UAX9]. However, rules for management and display of bidirectional
strings have been defined for domain name labels and similar
identifiers in the "Bidi Rule" specified in the IDNA2008
specification on right-to-left scripts [RFC5893].
The authors of a profile might believe that they need to define a new
directionality rule of their own. This is not a good idea, even if
the authors have done a great deal of careful research into the
problems of displaying bidirectional strings. Instead, this document
recommends the following:
o For profiles based on the IdentifierClass, it is best to use the
"Bidi Rule" specified in [RFC5893].
o For profiles based on the FreeformClass (or in situations where
the IdentifierClass is not appropriate), it is best to follow the
Unicode bidirectional algorithm [UAX9] directly.
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