[XeTeX] accented character ṛ within \section{ṛ}

[Arthur Reutenauer]

> Precomposed characters are defined to be canonically equivalent to the corresponding composite character sequences. Although it is not a requirement, software programs should treat them identically

  That's not exactly true.  The requirement is actually that programs
should not assume that two canonically equivalent sequences be treated
differently by other programs; but they may treat them differently
themselves.

	Arthur