autodiff

It's used only as a dev dependency in 4 crates.

Number of dependers	autodiff version	Downloads/month
3	0.7.0	280
1	0.6.0	5

Depender (with downloads and own rev deps)			autodiff version
		hrbf	dev	^0.7
		dfo	dev	^0.6
	2	meshx	dev	^0.7
		autodj	dev	^0.7.0