There are four compulsory ligatures for Arabic:
U+0644 U+0622 لآ
U+0644 U+0623 لأ
U+0644 U+0625 لإ
U+0644 U+0627 لا
Composing diacritics should work even inside the ligature:
U+0644 U+064E U+0627 لَا

For Urdu we introduced two more Yudit Ligatures in rel. 2.6.4.
U+06A9 U+200D U+0627 ک‍ا 
U+06A9 U+200D U+0644 ک‍ل

Composing diacritics should work even inside the ligature:
U+06A9 U+064E U+200D U+0627  کَ‍ا

Furthermore Yudit recognizes all optional ligatures that have
presentation forms in Unicode, like:
U+064A U+200D U+062D  ي‍ح
U+0645 U+200D U+062D U+200D U+0645 U+200D U+062F م‍ح‍م‍د

As defined in the Unicode Standard 3.1 (UAX 27), the
function of this character is to request a ligature
glyph if available, then fallback to cursively connected
characters and finally use the unconnected versions of
the letters.

Please note that the composition of these can be undone 
but delete and cursor movements will treat them as a whole.

Gaspar Sinai <gsinai@yudit.org>, Tokyo 2002-12-02
