vertex instanceOf

[augustearth]

Add support for declaring that a vertex is an instance of a class. Auto-create a relationship between instances and the class.

@VertexDefinition
static enum VX {
    A_CLASS,
    B_CLASS (
        superClass: VX.A_CLASS
    ),
    B (
        instanceOf: VX.B_CLASS
    )
}

When a B vertex is created, a IS_INSTANCE_OF relationship should be automatically created with the vertex representing B_CLASS.