rulebook-core/src/main/java/com/deliveredtechnologies/rulebook/FactMap.java
@Override
  public T getOne() {
    if (_facts.size() == 1) {
      return _facts.values().iterator().next().getValue();
    }
    return null;
  }
@Override
  public T getOne() {
    if (_facts.size() == 1) {
      return _facts.values().iterator().next().getValue();
    }
    return null;
  }
The method getOne() gets the value of the single Fact in the FactMap.
@return the value of a fact stored if only one fact is stored, otherwise null