Let A and C be two characters in a coded character sequence . C is blocked from A if and only if ccc(A)=0 and there exists some character B between A and C in the coded character sequence, i.e., , and either ccc(B)=0 or ccc(B) >= ccc(C).

• Because the Canonical Composition Algorithm operates on a string which is already in canonical order, testing whether a character is blocked requires looking only at the immediately preceding character in the string.