### Question:
DocumentRevisionTree { public Set<String> leafRevisionIds() { Set<String> res = new HashSet<String>(); for(DocumentRevisionNode obj : leafs()) { res.add(obj.getData().getRevision()); } return res; } DocumentRevisionTree();  DocumentRevisionTree(InternalDocumentRevision documentRevision); DocumentRevisionTree add(InternalDocumentRevision documentRevision); InternalDocumentRevision lookup(String id, String rev); int depth(long sequence); InternalDocumentRevision lookupChildByRevId(InternalDocumentRevision parentNode, String childRevision); InternalDocumentRevision bySequence(long sequence); boolean hasConflicts(); Map<Long, DocumentRevisionNode> roots(); DocumentRevisionNode root(long sequence); List<DocumentRevisionNode> leafs(); Set<String> leafRevisionIds(); List<InternalDocumentRevision> leafRevisions(); List<InternalDocumentRevision> leafRevisions(boolean excludeDeleted); InternalDocumentRevision getCurrentRevision(); List<InternalDocumentRevision> getPathForNode(long sequence); List<String> getPath(long sequence); String getDocId();  }### Answer:
@Test public void leafRevisions_emptyTree() { DocumentRevisionTree t = new DocumentRevisionTree(); Assert.assertThat(t.leafRevisionIds(), hasSize(0)); }