{
  test: {
    "readme.md": "hello world"
    "index.js" : "import nest from './a/nested.js'"
    a: {
      "nested.md": "myfile"
      "nested.js": "export default nest = ''"
      b: {
        "deeper.md" : "deep inside"
      }
    }
  }
  raw: ..
  files: Tree.filter(raw, (val, key) => key.endsWith('.md') && /^[a-p]+_/.test(key))
  regexpd: Tree.mask(test, Tree.regExpKeys({"^[^\/]+?\/.*\.md$": false}))
  //regexpdtop: Tree.mask(test, Tree.globKeys({"**": {"*.md": true}}))
  regexpdtest: Tree.mask(test, {
    ...Tree.constant(true) 
    ...Tree.regExpKeys({
    "^[^\/]+?\/.*\.md$": false
  })})
  regexpdfiles: Tree.mask(files, {
    ...Tree.constant(true) 
    ...Tree.regExpKeys({
    "^[^\/]+?\/.*\.md$": false
  })})
  paths: Tree.paths(test)
  regexpaths: Tree.map(Tree.filter(paths, (value) => /^[^\/]+?\/.*\.md$/.test(value)), {key: (value, key) => value, value: (value) => true})
  excluded: Tree.mask(paths, regexpaths)
}