grep { is_approximately_an_integer( @$_ ) } [
		poly_roots(
			poly_derivative(
                            
			# expanded form of x*(x - $x)*(x - $y)*(x - $z)
			1, -$x - $y - $z, $x*$y + $x*$z + $y*$z, -$x*$y*$z, 0
			)
		)       
	];