Reads all RDF quadruples
Input - files containing 1 RDF quadruple per line
Key: line number (ignored)
Value: an RDF quadruple
Output
Key: a pair of
cluster identifier and
subject or object of an RDF quadruple
Value: ignored
Reads RDF quadruples and outputs for each
quadruple where the predicate is rdf:type
the subjects of this type and an integer
for counting the appearances of the predicate.