When I try to do the path substact, I'v got this result:
In fact, my friends told me that it will also happen when doing other path calculations.
I'm a Fireworks user and extension developer, so here's my suggestion:
Fireworks has a 'Path' panel which is very powerful and useful, and you can get all the source code by Fireworks 'History' panel, just try any functions on the Path panel with selected paths, then choose the step on the History panel, click the 'Copy steps to clipboard' button on the bottom right, now you get the source code, just paste to any text editor you will see the code.
And there're also many extensions of Fireworks were open source on johndunning's website, all developers can borrow ideas from these Fireworks Gurus ( It's not stealing code from others, they're open source, which make Fireworks to be a real powerful screen design tool in the world until Adobe become it's stepmother ).
I really hope Affinity Deisgner become better, I'm not teaching you how to wirte the code, no doubt of it you guys are profession on codeing, but... you know, that's why open source exist, to spread all excellent ideas to others.
Just hope to help, when Affinity Designer comes out, come take my wallet :)
sorry for my poor english :(