function val = error_approx( x ) 
% error_approx - computes a good approximation to the error function.
a = abs(x) ; 
s = sign(x) ; 
val = s*(1 - 1/( 1 + (0.278393 + (0.230389 ... 
      + (0.000972 + 0.078108*a)*a)*a)*a)^4 );