Try with this :
rule "hello rule"
when
There is a Customer with first name "David"
then
Greet Customer
end
and this DSL :
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[condition][]There is a Customer with first name {name}=$customer :
Customer(firstName == {name})
[consequence][]Greet Customer=System.out.println("Hello " +
$customer.getFirstName());
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Of course your DSLR file is dependent of the DSL. Without the DSL, Drools cannot guess what you want to do.