Why use amount.token to initialize Paid variable? - unit-testing

Question

I am going through the Corda R3 training course and I am making headway, but when asked to create a Paid variable initialized to 0, the answer is:
package net.corda.training.state
import net.corda.core.contracts.Amount
import net.corda.core.contracts.ContractState
import net.corda.core.identity.Party
import java.util.*
/**
* This is where you'll add the definition of your state object. Look at the unit tests in [IOUStateTests] for
* instructions on how to complete the [IOUState] class.
*
* Remove the "val data: String = "data" property before starting the [IOUState] tasks.
*/
data class IOUState(val amount: Amount<Currency>,
val lender: Party,
val borrower: Party,
val paid: Amount<Currency> = Amount(0, amount.token) ):
ContractState {
override val participants: List<Party> get() = listOf()
}
Now I understand that we need to cast the value to type Amount, but why amount.token? I took the solution from:
https://github.com/corda/corda-training-solutions/blob/master/kotlin-source/src/main/kotlin/net/corda/training/state/IOUState.kt
Also, the task was to define it as Pounds, but I cannot figure out how to do so.
I find the reference for Pounds under:
https://docs.corda.net/api/kotlin/corda/net.corda.finance/kotlin.-int/index.html
I just do not understand how I would define the function.
Anyone have any pointers or suggestions for me? This code compiles and the tests pass, but I want to understand why... Thanks!

Answer 1

The token simply indicates what this is an amount of.
So when used here:
val paid: Amount<Currency> = Amount(0, amount.token)
You're taking whatever token was used for the amount parameter e.g. POUNDS, DOLLARS etc and setting the paid Amount to the same token type.
Take a look at how it's done in currencies.kt in Corda