The proof statements themselves, a set of triples (not quads)
Optional previousA collection of statements for a proof is to be in its own graph, generally with a blank node.
Note that the type restriction for this term is Quad_Subject, which stands for a term or a blank node, which is
more restrictive than a Quad_Graph, which may also have the value of a default graph. But proofs are always
real graphs.
Optional proofThe proof ID, which, in this implementation, is never a blank node, usually a UUID
The general structure for a Proof using n3.Store specifically; it also has a
perviousProofkey. This subclass is used when key chains or sets are extracted from an embedded proof.