code: public Collection<AbstractMockEc2Instance> describeInstances(final Set<String> instanceIDs) {
          if (null == instanceIDs || instanceIDs.size() == 0) {
              return allMockEc2Instances.values();
          } else {
              return getInstances(instanceIDs);
          }
      }
docs: List mock ec2 instances in current aws-mock.
  
  @param instanceIDs
  a filter of specified instance IDs for the target instance to describe
  @return a collection of {@link AbstractMockEc2Instance} with specified instance IDs, or all of the mock ec2
  instances if no instance IDs as filtered