@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...