code: public StatementGroup findStatementGroup(String propertyIdValue) {
  		if (this.claims.containsKey(propertyIdValue)) {
  			return new StatementGroupImpl(this.claims.get(propertyIdValue));
  		}
  		return null;
  	}
docs: Find a statement group by its property id, without checking for
  equality with the site IRI. More efficient implementation than
  the default one.