Incidentally, to be more complete concerning defines:  `defsubst'
defines an inline function.  The syntax is just like that of `defun'.
`defconst' defines a symbol as a constant.  The intent is that neither
programs nor users should ever change a value set by `defconst'.  (You
can change it; the value set is a variable; but please do not.)
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I can understant the last sentence in parentheses .
What does it mean ?
Thanks