org: tangle-mode

 
 14.8.2.22 ‘:tangle-mode’
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 The ‘tangle-mode’ header argument controls the permission set on tangled
 files.  The value of this header argument will be passed to
 ‘set-file-modes’.  For example, to set a tangled file as read only use
 ‘:tangle-mode (identity #o444)’, or to set a tangled file as executable
 shebang::) header arguments will automatically be made executable unless
 the ‘tangle-mode’ header argument is also used.  The behavior is
 undefined if multiple code blocks with different values for the
 ‘tangle-mode’ header argument are tangled to the same file.