rulebook-core/src/main/java/com/deliveredtechnologies/rulebook/lang/GivenRuleBuilder.java
public UsingRuleBuilder<T, U> using(String... factNames) {
    return new UsingRuleBuilder<T, U>(_rule, factNames);
  }
public UsingRuleBuilder<T, U> using(String... factNames) {
    return new UsingRuleBuilder<T, U>(_rule, factNames);
  }
Adds a using constraint in the Rule that restricts the facts supplied to the subsequent 'then' action.
@param factNames the fact names to be supplied to the subsequent 'then' action
@return          a builder the allows for the Rule to be built following the 'using' statement