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:
\input texinfo command
tells TeX to load the macros needed for processing a Texinfo file.
These are in a file called `texinfo.tex', which is usually located
in the `/usr/lib/tex/macros' directory. TeX uses the backslash,
`\', to mark the beginning of a command, just 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.