Excerpts from EuanM's message of 2015-12-09 01:59:43 +0100:
> http://www.unicode.org/reports/tr15/#Stable_Code_Points
> Table 7, the discussion of Ligatures, (which uses the ligature of
> "ffi" as its example)

ß is not a ligature of ss, but is a different character.
historically it evolved from a ligature of long s (ſ) and round s
but it is no no longer a true ligature that can be decomposed without sideeffects.

they are pronounced differently and there are german words where the difference
of using ß vs ss results in different meaning of the word: (eg Buße vs Busse:
penance vs busses)

https://en.wikipedia.org/wiki/ß

it is allowed to use ss as a replacement for ß only when ß itself is not available.

this is similar to the german umlauts: ä,ö and ü which can be decomposed into
ae, oe and ue, but those forms are a mere approximation, not equivalent. in a
medium where umlauts are available, using a decomposed form can be considered
an error.

greetings, martin.

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