Thank you for this information Suresh.

Suresh wrote:

  1. Please don't use ṁ for transcribing the half nasal. ṁ is used for anusvāra in ISO-15919. Sinhala should be using ISO-15919, not IAST. ṃ is not used in ISO-15919.
  2. ...
  3. m̐ should serve your purposes for representing the labial prenasalized consonant ඹ (U+0DB9 SINHALA LETTER AMBA BAYANNA)
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This is of course a workable work-around but the ISO 15919 standard for this letter is m with chandra over it not chandrabindu.  Suresh can you confirm if there is a unicode letter of m with chandra over it.  Is there a procedure to add new letters to Unicode.  I would think its reasonable that all ISO 15919 letters were included in Unicode as single glyphs.

Thanks,
Harry Spier