Chain children

What this field controls. Which rails or transfer templates may follow the chain's parent, and — for each — whether the child is a per-firing one-to-one follower or a many-to-one fan-in follower.

What you're looking at

A chip-list picker. Each chip is one selected child (rail or template), with a fan-in checkbox and an N integer slot next to it. The chip order is the canonical sequence the L1 invariants walk.

How children compose

The Z.A chain grammar reads the selection set:

• Exactly one chip selected. The child is required: every parent firing MUST be followed by an instance of this child within the parent's completion window. Missing follow-ups surface on the L1 (account-integrity) Exceptions sheet under the chain_parent_disagreement check-type branch.
• Two or more chips selected. The selection is an XOR alternation: exactly one of the selected children fires per parent firing. Any parent firing that matches more than one alternative, or matches zero, surfaces under the xor_group_violation or multi_xor_violation check-type branches on L1 Exceptions.

Empty selection is rejected — a chain with no children is meaningless.

Per-child fan-in (fan_in + expected_parent_count)

By default a chain child is one-to-one with its parent. Tick the fan-in checkbox on a chip when the child Transfer aggregates multiple parent firings — the canonical example is a merchant settlement cycle, where many transactions get netted into one settlement transfer.

The N slot to the right of the checkbox is the expected parent count (epc) — how many parent firings the validator expects per child Transfer. The L1 invariant fan_in_disagreement flags any child Transfer whose actual parent count doesn't match N.

Validator constraints:

• fan_in=true is only valid on TransferTemplate children. Rails fail validator C8a (TT-not-Rail gate).
• expected_parent_count must be ≥ 2 when fan_in=true (validator C8c). A fan-in of 1 is just one-to-one with a longer name.
• Mixed-cardinality is fine. Inside one XOR alternation, one child can be fan-in while the siblings stay one-to-one. The sasquatch MerchantSettlementCycle chain is the canonical demo.

When to reach for this

The chip-list version of this field exists because the validator's fan-in / epc rules are not derivable from the rest of the L2. The operator needs to declare them, and the form needs to keep them attached to the child chip — so unchecking fan-in surfaces a clean "this child is one-to-one" state instead of a stale epc=5 sitting under the surface.

Related handbook pages

• Transfer key — how leg firings group into the shared parent Transfer that the chain points at.
• Completion expression — the window during which the L1 layer expects the child to appear.

Vocabulary