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Characters and spaces in horizontal mode (TeX: The Program §1034–§1044, LuaTeX style).
In LuaTeX a character simply becomes a glyph node: ligatures and kerns
are applied later (see crate::nodeproc). The space factor is
maintained as in TeX82. Interword glue made from the font or from
\spaceskip has subtype spaceskip; glue from \xspaceskip has
subtype xspaceskip.