Splits a sheet scope that carries an external book prefix, or null if it carries none.
null
Quotes are the caller's problem: the scope reaching here has already been unquoted by the parser ('[1]P3.1'! and [1]P3.1! both arrive as [1]P3.1), which is the whole reason a book and its sheet are ONE symbol rather than two.
'[1]P3.1'!
[1]P3.1!
[1]P3.1
Splits a sheet scope that carries an external book prefix, or
nullif it carries none.