def format_fold_run(rep=None, fold=None, run=None, mode="concise"):
    """Construct a string to display the repetition, fold, and run currently being executed

    Parameters
    ----------
    rep: Int, or None, default=None
        The repetition number currently being executed
    fold: Int, or None, default=None
        The fold number currently being executed
    run: Int, or None, default=None
        The run number currently being executed
    mode: {"concise", "verbose"}, default="concise"
        If "concise", the result will contain abbreviations for rep/fold/run

    Returns
    -------
    content: Str
        A clean display of the current repetition/fold/run

    Examples
    --------
    >>> format_fold_run(rep=0, fold=3, run=2, mode="concise")
    'R0-f3-r2'
    >>> format_fold_run(rep=0, fold=3, run=2, mode="verbose")
    'Rep-Fold-Run: 0-3-2'
    >>> format_fold_run(rep=0, fold=3, run="*", mode="concise")
    'R0-f3-r*'
    >>> format_fold_run(rep=0, fold=3, run=2, mode="foo")
    Traceback (most recent call last):
        File "reporting.py", line ?, in format_fold_run
    ValueError: Received invalid mode value: 'foo'"""
    content = ""

    if mode == "verbose":
        content += format("Rep" if rep is not None else "")
        content += format("-" if rep is not None and fold is not None else "")
        content += format("Fold" if fold is not None else "")
        content += format("-" if fold is not None and run is not None else "")
        content += format("Run" if run is not None else "")
        content += format(": " if any(_ is not None for _ in [rep, fold, run]) else "")
        content += format(rep if rep is not None else "")
        content += format("-" if rep is not None and fold is not None else "")
        content += format(fold if fold is not None else "")
        content += format("-" if fold is not None and run is not None else "")
        content += format(run if run is not None else "")
    elif mode == "concise":
        content += format("R" if rep is not None else "")
        content += format(rep if rep is not None else "")
        content += format("-" if rep is not None and fold is not None else "")
        content += format("f" if fold is not None else "")
        content += format(fold if fold is not None else "")
        content += format("-" if fold is not None and run is not None else "")
        content += format("r" if run is not None else "")
        content += format(run if run is not None else "")
    else:
        raise ValueError("Received invalid mode value: '{}'".format(mode))

    return content