https://issues.dlang.org/show_bug.cgi?id=22299

          Issue ID: 22299
           Summary: RedBlackTree.empty missing "const" annotation
           Product: D
           Version: D2
          Hardware: All
               URL: http://dlang.org/
                OS: All
            Status: NEW
          Severity: major
          Priority: P3
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: eyal@weka.io

This does not compile:

    const(RedBlackTree!int) tree;
    assert(tree.empty); // "empty" not valid on a const object

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