       bsearch fileId key ?retvar? ?compare_proc?
              Search  an  opened  file fileId containing lines of
              text sorted into ascending order for a match.   Key
              contains  the string to match.  If retvar is speci-
              fied, then the line from the file  is  returned  in
              retvar, and the command returns 1 if key was found,
              and 0 if it wasn't.  If retvar is not specified  or
              is  a  null name, then the command returns the line
              that was found, or an empty string  if  key  wasn't
              found.

              By  default,  the  key is matched against the first
              white-space separated  field  in  each  line.   The
              field  is  treated  as  an  ASCII  string.  If com-
              pare_proc is specified, then it defines the name of
              a  Tcl procedure to evaluate against each line read
              from the sorted file during the  execution  of  the
              bsearch command.  Compare_proc takes two arguments,
              the key and a line extracted from  the  file.   The
              compare  routine  should  return a number less than
              zero if the key is less than the line, zero if  the
              key  matches  the line, or greater than zero if the
              key is greater than the line.   The  file  must  be
              sorted  in  ascending  order  according to the same
              criteria compare_proc uses to compare the key  with
              the line, or erroneous results will occur.

              This  command  does  not  work  on files containing
              binary data (bytes of zero).

              This command is provided by Extended Tcl.
