Add multiply-by-positive-constant primitive hmulpos.

[cheater]

Following your tips I have rewritten the "concatenation" function a little and it seems I have found a simple primitive that should provide stronger monotonicity.

varsAsVect xs = vOut
  -- | Take a list of scalar variables and output them as a single vector Expr
  where
    vOut = foldl' (+~) (head vs) (tail vs)
    xsIndices = zip ([0..] :: [Int]) xs
    vs = f <$> xsIndices
    f (ind, x) = v *~+ x
      where
        v = EConst $ orthonormal ind

    orthonormal i = assoc (n, 1) 0.0 [((i, 0), 1.0)]
    n = length xs

here *~+ is exactly the same as *~ except you can only have non-negative constants on the left, or it will throw an exception. Is my understanding correct that this requirement on the left argument will preserve monotonicity of the right argument?

[chrisnc]

I'm still skeptical about adding more exports. Also it seems unrelated to the other changes you're making.