The test suite doesn't work for me?!
I think it should be -2d not only -d:
assert(abs(sum([a b c].*(v1+[0 0 d/c]))-2d)<1e-4)
And it could be way more readable:
assert(abs(a*v1(1) + b*v1(2) + c*v1(3) - d) < 1e-4)
How did some people solve this?!?