Right now:

* Edge-Detect
	* Difference of Gaussians...
	* Edge...
		- Sobel        (type: gradient)
		- Prewitt      (type: compass)
		- Gradient     (type: gradient)
		- Roberts      (type: gradient)
		- Differential (type: gradient)
		- Laplace      (type: gradient usually used with
                                      Laplacian of Gaussian)
	* Laplace...
	* Neon...
	* Sobel...

What I suggest:

	* Edge-Detect
		* Difference of Gaussians...
		* Gradient...
			- Roberts
			- Sobel
			- Prewitt
			- Isotropic
		* Compass...
			- Sobel
			- Kirsch
			- Robinson
			- Prewitt
		* Laplace...
		* Neon...

Remove Laplace from edge.c (doesn't seem correct to me)
Merge Sobel.c in Gradient...