PackageDescr packageDescr = DescrFactory.newPackage().name("example")
                .newImport().target("java.math.BigDecimal").end()
                .newRule().name("Test Rule")
                    .lhs()
                        .accumulate()
                            .source().pattern().type("example.RuleTest.Fact").id("$target", false)
                            .end()
                        .end()
                    .function("count", "$cnt", false)
//  adding the following constraint can avoid from KieBase build error
//                 .constraint("true")
                    .end()
                .end() 
                .rhs("System.out.println($cnt);")
                .end()
            .end().getDescr();

        String drl = new DrlDumper().dump(packageDescr);

package example 

import java.math.BigDecimal

rule "Test Rule"
when
    accumulate(     
    $target : example.RuleTest.Fact(  )  ,
        $cnt : count(  ) ;
         ) 
then
System.out.println($cnt);

end