Normalization of Non-Zero Combining Marks

Q: Do all characters with a non-zero combining class have the "Maybe" quick check value?
A: No. There are several hundred characters in [\P{ccc=0}-\p{nfc_qc=maybe}]. Characters with a "Maybe" quick check are those that combine with a preceding starter into a composite character, which is not the case for all combining marks. The quick check algorithm involves checking not only the quick check flags but also checking the canonical order of characters with non-zero combining classes.

A primary composite must not have ccc!=0. (see D114: Primary Composite: A Canonical Decomposable Character (D69) which is not a Full Composition Exclusion)

Reason: A composite with ccc!=0 could cause normalization to produce un-normalized text. The problem is reordering, which is done before composition and where the composite's starter blocks reordering across it. After combining the starter with another character, if the composite had ccc!=0 it could be out of order compared to preceding or following characters, and there is not another reordering step to fix that. So this has to be forbidden.