Every Texinfo file that is to be the top-level input to TeX must begin
with a line that looks like this:
\input texinfo @c -*-texinfo-*-
This line serves two functions:
When the file is processed by TeX, the ‘\input texinfo’ command
tells TeX to load the macros needed for processing a Texinfo file.
These are in a file called texinfo.tex, which should have been
installed on your system along with either the TeX or Texinfo
software. TeX uses the backslash, ‘\’, to mark the beginning of
a command, exactly as Texinfo uses ‘@’. The texinfo.tex
file causes the switch from ‘\’ to ‘@’; before the switch
occurs, TeX requires ‘\’, which is why it appears at the
beginning of the file.
When the file is edited in GNU Emacs, the ‘-*-texinfo-*-’ mode
specification tells Emacs to use Texinfo mode.