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5.8.3 Auto-increment
--------------------

Registers can also be incremented or decremented by a configured amount
at the time they are interpolated.  The value of the increment is
specified with a third argument to the 'nr' request, and a special
interpolation syntax is used to alter and then retrieve the register's
value.  Together, these features are called "auto-increment".(1)  (*note
Auto-increment-Footnote-1::)

 -- Request: .nr ident value incr
     Set register IDENT to VALUE and its auto-incrementation amount to
     to INCR.  The '\R' escape sequence doesn't support an INCR
     argument.

   Auto-incrementation is not _completely_ automatic; the '\n' escape
sequence in its basic form never alters the value of a register.  To
apply auto-incrementation to a register, interpolate it with '\n±'.

 -- Escape sequence: \n+i
 -- Escape sequence: \n-i
 -- Escape sequence: \n+(id
 -- Escape sequence: \n-(id
 -- Escape sequence: \n+[ident]
 -- Escape sequence: \n-[ident]
     Increment or decrement IDENT (one-character name I, two-character
     name ID) by the register's auto-incrementation value and then
     interpolate the new register value.  If IDENT has no
     auto-incrementation value, interpolate as with '\n'.

     .nr a 0 1
     .nr xx 0 5
     .nr foo 0 -2
     \n+a, \n+a, \n+a, \n+a, \n+a
     .br
     \n-(xx, \n-(xx, \n-(xx, \n-(xx, \n-(xx
     .br
     \n+[foo], \n+[foo], \n+[foo], \n+[foo], \n+[foo]
         => 1, 2, 3, 4, 5
         => -5, -10, -15, -20, -25
         => -2, -4, -6, -8, -10

   To change the increment value without changing the value of a
register, assign the register's value to itself by interpolating it, and
specify the desired increment normally.  Apply an increment of '0' to
disable auto-incrementation of the register.