12.5.6 Glyph Substitution Tables
An object of type glyph-subst-table represents a one-to-one mapping from glyph-ids to glyph-ids.
(glyph-subst-table? obj)
Returns #t if obj is of type glyph-subst-table, and otherwise returns #f.
(glyph-subst-table list)
Returns an object of type glyph-subst-table. list shall contain a list of pairs of glyph-ids. In the resulting glyph-subst-table, the substitution for the first member of each pair is the second member. The substitution for any glyph-id that does not occur as the first member of a pair is itself. If a glyph-id occurs as the first member of more than one pair, then the substitution for that glyph-id is the second member of the first pair that has that glyph-id as its first member.
(glyph-subst glyph-subst-table glyph-id)
Returns the glyph-id that substitutes for glyph-id in the glyph-subst-table.