If you are not sure whether a given graph is created explicitly or not, in order to drop the graph use the DROP SILENT GRAPH
statement:
DROP SILENT GRAPH <http://example1.com> callret-0 Drop silent graph <http://example1.com> -- done
If a given graph is created explicitly however, i.e.
using the CREATE GRAPH
statement:
CREATE GRAPH <http://example2.com> callret-0 Create graph <http://example2.com> -- done
then you can use DROP GRAPH
in order to drop the graph:
DROP GRAPH <http://example2.com> callret-0 Drop graph <http://example2.com> -- done