def _canonicalize_units(units):
    """
    Standardized units to a lower case representation. 
    
    Args: 
        units: A list of strings
        
    Returns:
        A dictionary that maps a lower case unit to its corresponding unit.
    """
    unit_dict = {}
    for unit in units:
        unit_dict[unit.lower()] = unit 
    return unit_dict