kpathsea: Brace expansion
5.3.4 Brace expansion
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'x{A,B}y' expands to 'xAy:xBy'. For example:
foo/{1,2}/baz
expands to 'foo/1/baz:foo/2/baz'. ':' is the path separator on the
current system; e.g., on a Windows system, it's ';'.
Braces can be nested; for example, 'x{A,B{1,2}}y' expands to
'xAy:xB1y:xB2y'.
Multiple non-nested braces are expanded from right to left; for
example, 'x{A,B}{1,2}y' expands to 'x{A,B}1y:x{A,B}2y', which expands to
'xA1y:xB1y:xA2y:xB2y'.
This feature can be used to implement multiple TeX hierarchies, by
assigning a brace list to '$TEXMF', as mentioned in 'texmf.in'.
You can also use the path separator instead of the comma. The last
example could have been written 'x{A:B}{1:2}y' (on Unix).
Brace expansion is implemented in the source file
'kpathsea/expand.c'.