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";
}