If  an argument appears to be an ASCII file, file attempts
       to guess its language.  The language tests look  for  par­
       ticular  strings  (cf names.h) that can appear anywhere in
       the first few blocks of a file.  For example, the  keyword
       .br  indicates  that  the  file  is most likely a troff(1)
       input file, just as the keyword struct indicates a C  pro­
       gram.  These tests are less reliable than the previous two
       groups, so they are performed  last.   The  language  test
       routines  also  test  for  some miscellany (such as tar(1)
       archives) and determine whether an unknown file should  be
       labelled as `ascii text' or `data'.