What is a Subject?

A Subject is a self-descriptive value type with three fields:

• Identity (Symbol): an optional identifier, e.g. "alice". Used to look up a pattern in a StandardGraph.
• Labels (Set<String>): zero or more string labels, e.g. ["Person", "Employee"].
• Properties (Map<String, Value>): zero or more named properties with typed values.

Subject { identity: "alice", labels: ["Person"], properties: { age: 30 } }

Pattern<Subject> is the standard type for property-graph data in this library. Each position in a pattern carries a Subject that describes what that element is: a node, an endpoint of a relationship, a relationship itself.

Subject implements Combinable — two Subject values can be merged. The result keeps the first identity, unions the labels, and merges the properties (first value wins on conflict). This makes it natural to refine a subject by combining partial descriptions.

A minimal subject can be created with just an identity and then augmented:

let s = Subject::from_id("alice")
    .with_label("Person")
    .with_property("age", Value::Int(30));