#
# This software is Copyright 2005 by Elsevier Inc.  You may use it
# under the terms of the license at http://perl.plover.com/hop/LICENSE.txt .
#



###
### slope
###

## Chapter 7 section 1

sub slope {
  my $f = shift;
  my $e = 0.00000095367431640625;
  my $d = sub {
    my ($x) = shift;
    ($f->($x+$e) - $f->($x-$e)) / (2*$e);
  };
  return @_ ? $d->(shift) : $d;
}