Problem of explicit Virama (devanagari)
 
 Unicode 5.0 page 305 R16 starts as this:
 
 The presence of an explicit virama (either caused by a ZWNJ or by the absence of a conjunct in the font) blocks this reordering, and the dependent vowel Ivs is rendered after the rightmost such explicit virama.
 
 Strategy of the FLT for Devanagari
  Apply OTF features half and pres to the pre-base and base consonants.
  If the resulting glyph is one, the font has a conjunct form.