def min_delta(delta):
    """
    Minimum delta criteria

    Parameters
    ----------
    delta : float
        The minimum height of a leaf above its merger level

    """
    def result(structure, index=None, value=None):
        if value is None:
            if structure.parent is not None:
                return (structure.height - structure.parent.height) >= delta

            return (structure.vmax - structure.vmin) >= delta
        return (structure.vmax - value) >= delta
    return result