# Register Ledger

Function:

```
fun register_ledger(
    registry: &mut LedgerRegistry,
    collection_id: ID,
    currency_type: TypeName,
    ledger_id: ID
)

```

**Description:**

* Adds a **trade ledger reference** to the registry.
* Ensures that **each collection has only one ledger per currency**.

**Validations:**

* A ledger must **not already exist** for the collection.