success = ExtendFont(font, fontTags)
D0 A0 A1
ULONG ExtendFont(struct TextFont *, struct TagItem *);
success = ExtendFontTags(font, Tag1, ...) (V39)
ULONG ExtendFontTags(struct TextFont *, ULONG, ...);
To extend a TextFont structure.
The varargs stub was missing from amiga.lib until V39.