shapes-ISSUE-56 (labra): Inverse triple constraints [SHACL Spec]

shapes-ISSUE-56 (labra): Inverse triple constraints [SHACL Spec]

http://www.w3.org/2014/data-shapes/track/issues/56

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

A common use case is to define constraints on the incoming arcs of a node. In compact syntax, inverse constraints are defined by the "^" operator. 

For example:

<IsKnown> {
 ^foaf:knows @<PersonShape>+
}

defines nodes that receive one or more arcs "foaf:knows" from a node of shape <PersonShape>

The SHACL spec should define the semantics of inverse triple constraints and its interaction with other features of the language.