Enum SubgraphPattern 

pub enum SubgraphPattern {
    Declarative(PatternTree),
    Callable(fn(&Graph, NodeId) -> Option<PatternMatch>),
}

Canonical primitive-subgraph pattern that a fused op represents. Used by fusion rules.

Two reasonable shapes; the registry supports both:

• Callable — a closure-style matcher, mirrors PR 3’s canonical_softmax_pattern. Maximally flexible (consumer-count guards, cross-checks); less analyzable.
• Declarative — a recursive PatternTree that the rule engine compiles to a matcher. More analyzable; auto-generation from the registry entry is straightforward.

Step 3 ports SoftmaxLastDim’s existing matcher to Callable. The declarative form (see Q1 in docs/fused-op-registry.md) is wired up in step 4 once a second op exercises the abstraction.

Variants

Declarative(PatternTree)

Callable(fn(&Graph, NodeId) -> Option<PatternMatch>)