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EVA is the BRIK64 composition model. It gives agents and developers three ways to explain how circuit units connect.
CompositionMeaningAgent question
SequentialA runs before B and B uses A’s output.What data flows from A to B?
ConditionalOne branch is selected by a condition.Do all branches return the same kind of result?
ParallelIndependent branches are evaluated separately.Are the branches independent?

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Sequential

Sequential composition is the normal pipeline shape. 
PC label_hash {
    fn label_hash(prefix, code) {
        let label = MC_40.CONCAT(prefix, code);
        let digest = MC_48.HASH(label);
        return digest;
    }
}
The output of CONCAT feeds HASH.

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Conditional

Conditional composition must close every branch. 
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.

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Parallel

Use parallel composition only when branches are independent. In PCD examples, represent this as separate values that do not mutate shared state. 
PC parallel_summary {
    fn parallel_summary(left, right) {
        let left_len = MC_43.LEN(left);
        let right_len = MC_43.LEN(right);
        let total = MC_00.ADD8(left_len, right_len);
        return total;
    }
}

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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.