In groff, Heirloom Doctools troff, and neatroff, the `\~` escape
sequence produces an adjustable (paddable) but unbreakable space.
mandoc, which does not perform adjustment or render to typesetters,
supports the escape sequence as a synonym for `\ `, the same as `\0`.
This extension is of long pedigree: groff has supported it for at least
30 years, Heirloom for 17, mandoc for 13, and neatroff for 6.
Do the same as mandoc to prevent mis-rendering of documents using this
escape sequence. Heirloom Doctools troff, a descendant of Documenter's
Workbench troff, would be a good place to look for a compatible
implementation of the adjustable semantics for this sequence.
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