function d = p06_fd ( p )

%*****************************************************************************80
%
%% p06_fd() is a signed distance function for problem 06.
%
%  Modified:
%
%    06 February 2006
%
%  Input:
%
%    real P, one or more points.
%
%  Output:
%
%    real D, the signed distance of each point to the boundary of the region.
%
  d = ddiff ( dexpr ( p, '(x^4+y^4)^(1/4)-1' ), ...
              dexpr ( p, '(x^4+y^4)^(1/4)-0.5' ) );

  return
end
