This is the file .../info/dir, which contains the
topmost node of the Info hierarchy, called (dir)Top.
The first time you invoke Info you start off looking at this node.

File: dir,	Node: Top	This is the top of the INFO tree

  This (the Directory node) gives a menu of major topics.
  Typing "q" exits, "?" lists all Info commands, "d" returns here,
  "h" gives a primer for first-timers,
  "mEmacs<Return>" visits the Emacs manual, etc.

  In Emacs, you can click mouse button 2 on a menu item or cross reference
  to select it.

* Menu:

Software libraries
* gsl-ref: (gsl-ref).           GNU Scientific Library - Reference

Texinfo documentation system
* fftw3: (fftw3).               FFTW User's Manual.