Question

There are some characters which are not included in Unicode(i.e. accented Cyrillic letters) but can be created using combining sequences. As I understand the possible combining character sequences are defined in the layout engine and/or font used. Am I right? So, how to get all the possible combining sequences?

Answer 1

You are correct in that attempting to create arbitrary combining sequences may fail for a combination of layout engine and font. A solution to this problem is outside the remit of the Unicode standard.

From Unicode 6.2, chapter 2:

All combining characters can be applied to any base character and can, in principle, be used with any script. As with other characters, the allocation of a combining character to one block or another identifies only its primary usage; it is not intended to define or limit the range of characters to which it may be applied. In the Unicode Standard, all sequences of character codes are permitted.

This does not create an obligation on implementations to support all possible combinations equally well. Thus, while application of an Arabic annotation mark to a Han character or a Devanagari consonant is permitted, it is unlikely to be supported well in rendering or to make much sense.