The file src/approx.c is copied from the stats package that is part of 
base R. This particular version has been copied from R-2.15.1. 
The following statement applies to this file:

  R : A Computer Language for Statistical Data Analysis
  Copyright (C) 1995-2001   Robert Gentleman, Ross Ihaka and the
                            R Core Team

  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or
  (at your option) any later version.
 
  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, a copy is available at
  http://www.r-project.org/Licenses/