Centered hyphens are entered as `--' between syllables.
The hyphen will have variable length depending on the space between
the syllables and it will be centered between the syllables.
When a lyric is sung over many notes (this is called a melisma), this is
indicated with a horizontal line centered between a syllable and the
next one. Such a line is called an extender line, and it is entered as
__.
Program reference: HyphenEvent, ExtenderEvent, LyricHyphen, and LyricExtender
Examples: input/test/lyric-hyphen-retain.ly.
This page is for LilyPond-2.4.5 (stable-branch).