@Jonathan -

I don't think this is quite right.

For one, we want the following equation to be true:

$$
2 H + O \to H_2O
$$

But we also want

$$
H \to H
$$

And we should be able to add the two equations together (as per the monoidal preorder rule), hence:

$$
3 H + O \to H + H_2O
$$

But that doesn't fit the scheme you and Marius propose...