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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 serial_load(workflow_file, specification, parameters=None, original=None):
 """Validate and return a expanded REANA Serial workflow specification. :param workflow_file: A specification file compliant with REANA Serial workflow specification. :returns: A dictionary which represents the valid Serial workflow with all parameters expanded. """

    parameters = parameters or {}

    if not specification:
        with open(workflow_file, 'r') as f:
            specification = json.loads(f.read())

    expanded_specification = _expand_parameters(specification,
                                                parameters,
                                                original)

    validate(specification, serial_workflow_schema)

    return expanded_specification