About: http://webisa.webdatacommons.org/30648311

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	So if the count is 0, you know that @y is a subset of @x and hence you want to return it from the induced subroutine.
http://www.w3.org/ns/prov#wasQuotedFrom	perlmonks.org