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[spec] Multiple substitution with classes syntax #1651

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[spec] Multiple substitution with classes syntax #1651

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27 changes: 23 additions & 4 deletions docs/OpenTypeFeatureFileSpecification.md
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Expand Up @@ -1602,18 +1602,37 @@ Omitting the `by` clause is equivalent to adding `by NULL`.
<a name="5.b"></a>
### 5.b. [GSUB LookupType 2] Multiple substitution

A Multiple Sub rule is specified as:
A Multiple Sub rule is specified in one of the following formats:

```fea
substitute <glyph> by <glyph sequence>;
substitute <glyph> by <glyph sequence>; # Format A
substitute <glyphclass> by <glyph sequence>; # Format B
substitute <glyphclass> by <glyphclass sequence>; # Format C
```

Format A specifies that target glyph is replaced by the replacement sequence.
`<glyph sequence>` contains two or more glyphs. It may not contain glyph
classes. (If it did, the rule would be ambiguous as to which replacement
sequence were required.) For example:
sequence were required.)

Format B specifies that any glyph in the target glyph class must be replaced by
the same replacement glyph sequence.

Format C specifies that any glyph in the target glyph class must be replaced by
corresponding glyph (in the order of glyphs in the glyph classes) in the
replacement sequence. `<glyph sequence>` contains two or more glyph classes. If
the replacement sequence contains single glyphs or singleton glyph classes,
they will be repeated for each glyph in the target glyph class, otherwise if
the replacement glyph classes has more than one glyph, then the number of
elements in the target and replacement glyph classes must be the same.

For example:

```fea
substitute f_f_i by f f i; # Ligature decomposition
substitute f_f_i by f f i; # Format A
substitute [f_i fi] by f i; # Format B
substitute [f_i f_l T_h] by [f f T] [i l h]; # Format C
substitute [f_i f_l] by f [i l]; # Format C
```

<a name="5.c"></a>
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