We can use HTML lists to specify tree values. The class representing the tree implements the fitlibrary.tree.Tree interface.

using fitlibrary.tree;

public class SampleDomain {
    public Tree MakeTree(string[] leaves) {...}
}

when test is⇓ story test fixture check test result sample domain check make tree a,b a b check make tree blank sample domain check make tree a,b a b check make tree blank

sample domain check make tree a,b a b check make tree blank

then result is

sample domain check make tree a,b a b check make tree blank