> ## Documentation Index
> Fetch the complete documentation index at: https://docs.brik64.com/llms.txt
> Use this file to discover all available pages before exploring further.

# EVA Tutorial

> Use EVA to reason about BRIK64 composition.

EVA is the BRIK64 composition model. It gives agents and developers three ways to explain how circuit units connect.

| Composition | Meaning                                        | Agent question                                  |
| ----------- | ---------------------------------------------- | ----------------------------------------------- |
| Sequential  | A runs before B and B uses A's output.         | What data flows from A to B?                    |
| Conditional | One branch is selected by a condition.         | Do all branches return the same kind of result? |
| Parallel    | Independent branches are evaluated separately. | Are the branches independent?                   |

## Sequential

Sequential composition is the normal pipeline shape.

```pcd theme={null}
PC fee_total {
    fn fee_total(amount: i64, fee: i64) -> i64 {
        return MC_00.ADD8(amount, fee);
    }
}
```

The amount and fee inputs feed a named numeric operation.

## Conditional

Conditional composition must close every branch.

```pcd theme={null}
PC order_gate {
    fn order_gate(amount, limit) {
        if (amount <= 0) {
            return 0;
        }

        if (amount > limit) {
            return 0;
        }

        return 1;
    }
}
```

Every path returns a value, and the blocked paths are explicit.

## Parallel

Use parallel composition only when branches are independent. In PCD examples, represent this as separate values that do not mutate shared state.

```pcd theme={null}
PC parallel_summary {
    fn parallel_summary(left: i64, right: i64) -> i64 {
        return MC_00.ADD8(left, right);
    }
}
```

## Authoring Guidance

Do not invent symbolic EVA syntax in executable PCD. Use source structure and topology metadata unless a specific compiler release documents a supported symbolic form.