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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 map_to_openapi_type(self, *args):
 """Decorator to set mapping for custom fields. ``*args`` can be: - a pair of the form ``(type, format)`` - a core marshmallow field type (in which case we reuse that type's mapping) """

        if len(args) == 1 and args[0] in self.field_mapping:
            openapi_type_field = self.field_mapping[args[0]]
        elif len(args) == 2:
            openapi_type_field = args
        else:
            raise TypeError("Pass core marshmallow field type or (type, fmt) pair.")

        def inner(field_type):
            self.field_mapping[field_type] = openapi_type_field
            return field_type

        return inner