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5.6.1 Control Characters
------------------------

The mechanism of using 'roff''s control characters to invoke requests
and call macros was introduced in *note Requests and Macros::.  Control
characters are recognized only at the beginning of an input line, or at
the beginning of the branch of a control structure request; see *note
Conditionals and Loops::.

   A few requests cause a break implicitly; use the no-break control
character to prevent the break.  Break suppression is its sole
behavioral distinction.  Employing the no-break control character to
invoke requests that don't cause breaks is harmless but poor style.
*Note Manipulating Filling and Adjustment::.

   The control '.' and no-break control ''' characters can each be
changed to any ordinary character(1) (*note Control
Characters-Footnote-1::) with the 'cc' and 'c2' requests, respectively.

 -- Request: .cc [o]
     Recognize the ordinary character O as the control character.  If O
     is absent or invalid, the default control character '.' is
     selected.  The identity of the control character is associated with
     the environment (*note Environments::).

 -- Request: .c2 [o]
     Recognize the ordinary character O as the no-break control
     character.  If O is absent or invalid, the default no-break control
     character ''' is selected.  The identity of the no-break control
     character is associated with the environment (*note
     Environments::).

   When writing a macro, you might wish to know which control character
was used to call it.

 -- Register: \n[.br]
     This read-only register interpolates 1 if the currently executing
     macro was called using the normal control character and 0
     otherwise.  If a macro is interpolated as a string, the '.br'
     register's value is inherited from the context of the string
     interpolation.  *Note Strings::.

     Use this register to reliably intercept requests that imply breaks.

          .als bp*orig bp
          .de bp
          .  ie \\n[.br] .bp*orig
          .  el          'bp*orig
          ..

     Testing the '.br' register outside of a macro definition makes no
     sense.