rulebook-core/src/main/java/com/deliveredtechnologies/rulebook/lang/RuleBuilder.java
@SuppressWarnings("unchecked")
  public UsingRuleBuilder<T, U> using(String... factNames) {
    Rule<T, U> rule = _name.map(name -> (Rule<T, U>)new AuditableRule<T, U>(newRule(), name)).orElse(newRule());
    if (rule == null) {
      throw new IllegalStateException("No Rule is instantiated; An invalid Rule class may have been provided");
    }
    return new UsingRuleBuilder<T, U>(rule, factNames);
  }
@SuppressWarnings("unchecked")
  public UsingRuleBuilder<T, U> using(String... factNames) {
    Rule<T, U> rule = _name.map(name -> (Rule<T, U>)new AuditableRule<T, U>(newRule(), name)).orElse(newRule());
    if (rule == null) {
      throw new IllegalStateException("No Rule is instantiated; An invalid Rule class may have been provided");
    }
    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