@Override
    public TensorType type(TypeContext<Reference> context) {
        TensorType type;
        try {
            type = context.getType(reference);
        }
        catch (IllegalArgumentException e) {
            throw new IllegalArgumentException(reference + " is invalid", e);
        }
        if (type == null)
            throw new IllegalArgumentException("Unknown feature '" + this + "'");
        return type;
    }
Returns the reference of this node
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/rule/ReferenceNode.java