Yes thank you, but accidentally typed in the wrong amount.
Am I correct that this pair or recipes would do and undo correctly?
<set xpath="/recipes/recipe[@name='ammoGasCan'][@count='600']/@count">1200</set>
<set xpath="/recipes/recipe[@name='oilBarrel']/ingredient[@name='ammoGasCan']/@count">1200</set>
I might have done something wrong, It still seems to be using 600 units of gas both ways in the recipe.