shapes-ISSUE-55 (labra): Triple constraints [SHACL Spec]

shapes-ISSUE-55 (labra): Triple constraints [SHACL Spec]

http://www.w3.org/2014/data-shapes/track/issues/55

Raised by: Jose Emilio Labra Gayo
On product: SHACL Spec

One of the most basic constraint is to constraint an outgoing arc from a node. 

In compact syntax, this constraint is usually called a triple constraint. An example can be:

<Labelled> {
 rdfs:label xsd:string
}

SHACL spec should define the semantics of this constraint and the interaction between triple constraints and open/closed shapes.