fonttools/Tests/feaLib/data/spec5f_ii_2.fea

# OpenType Feature File specification, section 5.f.ii, example 2
# "Specifying exceptions to the Chain Sub rule"
# http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html

feature test {
    @LETTER = [a - z];
    ignore sub @LETTER f' i';
    sub f' i' by f_i.begin;
} test;