How can I apply a minus operation to Tez type in the archetype testing library?

Question

I am writing some simple contract tests, and I want to check that account balances are updating appropriately.

There is currently no minus method on the Tez type. I attempted to add my own to the module, intending to make a PR if it worked - adding the following method to the Tez class in main.ts of archetype-ts-types...

minus(x: Tez): Tez {
    const result = this._content.minus(x.to_big_number())
    if (this._content.isLessThan(new BigNumber(0)) || this._content.isGreaterThan(new BigNumber("")))
          throw new Error("Invalid Tez value")
    return new Tez(result, "mutez")
  }

However I simply get the error "Property minus does not exist on type Tez".

Really I have two related questions -

1. Why don't my updates to the types library in my local files flow through to my test script?
2. Is this the right track or is there actually already a better way of doing this?

(I realise I can add "plus" to the other side of the assert equation, which is what I am doing for now, but it makes it easier to follow the meaning of the tests code having the minus option, so that would handy)

Help appreciated - thanks

Answer 1

1. because Typescript is compiled to Js; what is executed is the precompiled Js; changing the source won't change the precompiled Js.
2. you could use the to_big_number method:

const a = new Tez(10)
const b = new Tez(25)
const c = a.to_big_number().minus(b.to_big_number())
console.log(c.toNumber())

the above prints -15000000