def consistent_dims(value_items):
    """
    Get dimensions used consistently across all value objects.
    Returns None if dimensions are omitted or added for
    some value object(s).
    """
    used = set(k for k in value_items[0] if k != "value")
    for vo in value_items:
        if used != set(k for k in vo if k != "value"):
            return None
    return used