def builtin_to_string(builtin_type):
    """
    Given a builtin type, return its corresponding string representation.
    """
    if is_dict(builtin_type):
        return "dict"
    return _TYPES_TO_STRINGS[builtin_type]

    Given a builtin type, return its corresponding string representation.
    
coremltools/converters/mil/mil/types/type_mapping.py