(m4.info)Incr

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12.1 Decrement and increment operators
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Increment and decrement of integers are supported using the builtins
'incr' and 'decr':

 -- Builtin: incr (NUMBER)
 -- Builtin: decr (NUMBER)
     Expand to the numerical value of NUMBER, incremented or
     decremented, respectively, by one.  Except for the empty string,
     the expansion is empty if NUMBER could not be parsed.

     The macros 'incr' and 'decr' are recognized only with parameters.

     incr(`4')
     =>5
     decr(`7')
     =>6
     incr()
     error->m4:stdin:3: empty string treated as 0 in builtin `incr'
     =>1
     decr()
     error->m4:stdin:4: empty string treated as 0 in builtin `decr'
     =>-1