Nested \discretionary-ies in (Xe)LaTex resulting an error

Question

I want to set an automatic hyphenation for the Armenian word յանդգնել in XeLaTeX with \discretionary. The problem is, that there are four consonants in a row, and the Armenian language has very strange (for English speakers) rules about hyphenating such words. That word can hyphenated in two ways: յան֊դըգնել and յանդըգ֊նել, in both of them the letter ը reappears where it sounds. This can be correctly achieved with

յան\h{֊}{դըգ}{դ\h{ըգ֊}{}{գ}}նել,

where I've defined \h as

\newcommand{\h}[3]{\discretionary{#1}{#2}{#3}},

but there are nested \h-s here, so it gives an error

Improper discretionary list. ... յան\h{֊}{դըգ}{դ\h{ըգ֊}{}{գ}}

and the compiler (XeLaTex) just removes the second \h where it is needed.

I've found nothing about this topic in the Internet, and, unfortunately, there is no automatic hyphenation package for the Armenian language. What's wrong in nested \discretionary-ies?

P.S. Armenian uses another sign for hyphenation (a slightly curved ֊, not -), so I need to use \discretionary anyway.

Answer 1

As you've seen nested \discretionaryies are not supported in TeX but as a workaround you can use that in most documents every line will have more than five characters, therefore it is unlikely that both hyphenation opportunities are taken. So usually you can archive your goal with

յան\h{֊}{դը}{դ}\h{ըգ֊}{}{գ}նել

Of course this technically allows the (wrong) hyphenation

յան֊
դըըգ֊
նել

but under normal linelengths this is unlikely to be chosen because "դըըգ֊" is far from filling a line.

If you don't want to make such a compromise, you need LuaTeX. While LuaTeX doesn't support nested discretionaries in general either, it does support one special case which should be applicable here: Use

\newcommand\weirddisc[6]{%
  \discretionary{#1}{#2}{#3}\discretionary{#4}{#5}{#6}%
  \directlua{
    local last = tex.nest.top.tail
    last.subtype = 5
    last.prev.subtype = 4
  }}
...
յան\weirddisc{֊}{դըգ}{դգ}{դըգ֊}{}{դըգ-}նել

Here

\weirddisc{1}{2}{3}{4}{5}{6}

represents the "nested discretionaries"

\h{1}{\h{4}{5}{2}}{\h{6}{5}{3}}