erf(x,y)

[cossio]

Would people agree on adding a two-argument erf(x, y) = erf(y) - erf(x)? It could be implemented like this:

function erf(x, y)
	if x ≤ 0 ≤ y || y ≤ 0 ≤ x
		erf(y) - erf(x)
	elseif 0 ≤ x && 0 ≤ y
		erfc(x) - erfc(y)
	elseif x ≤ 0 && y ≤ 0
		erf(-y, -x)
	end
end

avoiding catastrophic cancellation. I use this frequently so I could do a PR.