code: func (conf *LiveIndexWriterConfigImpl) SetMergePolicy(mergePolicy MergePolicy) *LiveIndexWriterConfigImpl {
  	assert2(mergePolicy != nil, "mergePolicy must not be nil")
  	conf.mergePolicy = mergePolicy
  	return conf
  }
docs: /*
  Expert: MergePolicy is invoked whenver there are changes to the
  segments in the index. Its role is to select which merges to do, if
  any, and return a MergeSpecification describing the merges. It also
  selects merges to do for forceMerge.
  */