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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _FetchCustomMappings(descriptor_ls):
 """Find and return all custom mappings for descriptors in descriptor_ls."""

    custom_mappings = []
    for descriptor in descriptor_ls:
        if isinstance(descriptor, ExtendedEnumDescriptor):
            custom_mappings.extend(
                _FormatCustomJsonMapping('Enum', m, descriptor)
                for m in descriptor.enum_mappings)
        elif isinstance(descriptor, ExtendedMessageDescriptor):
            custom_mappings.extend(
                _FormatCustomJsonMapping('Field', m, descriptor)
                for m in descriptor.field_mappings)
            custom_mappings.extend(
                _FetchCustomMappings(descriptor.enum_types))
            custom_mappings.extend(
                _FetchCustomMappings(descriptor.message_types))
    return custom_mappings