code: public ICondition getEffectiveCondition()
      {
      if( effCondition_ == null)
        {
        effCondition_ =
          getEffectiveCondition(
            Optional.ofNullable( getParent())
            .map( IVarDef::getEffectiveCondition)
            .orElse( null));
        }
      
      return effCondition_;
      }
docs: Returns the effective condition that defines when this variable is applicable,
  based on the conditions for this variable and all of its ancestors.