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5.12.1 Leaders
--------------

Sometimes it is desirable to fill a tab stop with a given glyph, but
also use tab stops normally on the same output line.  An example is a
table of contents entry that uses dots to bridge the entry name with its
page number, which is itself aligned between tab stops.  The 'roff'
language provides "leaders" for this purpose.(1)  (*note
Leaders-Footnote-1::)

   A leader character (ISO and EBCDIC code point 1, also known as SOH or
"start of heading"), behaves similarly to a tab character: it moves to
the next tab stop.  The difference is that for this movement, the
default fill character is a period '.'.

 -- Escape sequence: \a
     Interpolate a leader in copy mode; see *note Copy Mode::.

 -- Request: .lc [c]
     Set the leader repetition character to the ordinary or special
     character C.  Recall *note Tabs and Leaders::: when encountering a
     leader character in the input, the formatter writes as many dots
     '.' as are necessary until reaching the next tab stop; this is the
     "leader definition character".  Omitting C unsets the leader
     character.  With no argument, GNU 'troff' treats leaders the same
     as tabs.  The leader repetition character is associated with the
     environment (*note Environments::).  Only a single C is recognized;
     any excess is ignored.

   A table of contents, for example, may define tab stops after a
section number, a title, and a gap to be filled with leader dots.  The
page number follows the leader, after a right-aligned final tab stop
wide enough to house the largest page number occurring in the document.

     .ds entry1 19.\tThe Prophet\a\t98
     .ds entry2 20.\tAll Astir\a\t101
     .ta .5i 4.5i +.5iR
     .nf
     \*[entry1]
     \*[entry2]
         => 19.  The Prophet.............................   98
         => 20.  All Astir...............................  101