fonttools/Tests/feaLib/data/spec5f_ii_4.fea

# OpenType Feature File specification, section 5.f.ii, example 4
# "Specifying exceptions to the Chain Sub rule"
# http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html

@LETTER = [A - Z a - z];

feature cswh {

    # --- Glyph classes used in this feature:
    @BEGINNINGS = [A - N P - Z T_h m];
    @BEGINNINGS_SWASH = [A.swash - N.swash P.swash - Z.swash T_h.swash m.begin];
    @ENDINGS = [a e z];
    @ENDINGS_SWASH = [a.end e.end z.end];

    # --- Beginning-of-word swashes:
    ignore sub @LETTER @BEGINNINGS';
    sub @BEGINNINGS' by @BEGINNINGS_SWASH;
 
    # --- End-of-word swashes:
    ignore sub @ENDINGS' @LETTER;
    sub @ENDINGS' by @ENDINGS_SWASH;

} cswh;