=item calculate_area

The calculate_area function calculates the area of the
points passed in.  You can pass the points in a number of
different ways, but, regardless, you'll get the area back.

=over 4

=item *

You can pass in a series of points.  The function will
assume that the shape described is a closed-shape, that is,
the first point is automatically repeated as the end point.
You can pass in the points as perl6 pairs, e.g.:

    $area = calculate_area( $x0 => $y0, $x1 => $y1 );

or as two lists of x's and y's:

    $area = calculate_area( @x, @y );

or as a list of Point objects

    $area = calculate_area( @points );

=item *

You can pass in a Shape object.

    $area = calculate_area( $shape );

=item *

You can pass in an equation, and a beginning x-value and
an ending x-value, and the area will be that under the 
curve.  The area may be negative in this case.

    $area = calculate_area( $equation, $x0, $x1 );

=back

Returns: the area of the shape, in square units.

=cut

##</code><code>##

sub calculate_area
{
  unless ( grep { ref $_ ne 'Pair' } @_ )
  { goto \&_calculate_area_pairs; }
  if (@_ % 2 == 0 and not ref $_[0])
  { goto \&_calculate_area_x_list_y_list; }
  unless ( grep { ref $_ ne 'Point' } @_ )
  { goto \&_calculate_area_list_of_points; }
  unless ( grep { ref $_ ne 'ARRAY' } @_ )
  { goto \&_calculate_area_list_of_arrays; }
  if (UNIVERSAL::isa($_[0], 'Shape'))
  { goto \&_calculate_area_share; }
  if (UNIVERSAL::isa($_[0], 'Equation') and @_ == 3)
  { goto \&_calculate_area_equation; }

  die "Didn't recognise how you called calculate_area";
}