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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 validate(self, processes=1, fast=False, completeness_only=False, callback=None):
 """Checks the structure and contents are valid. If you supply the parameter fast=True the Payload-Oxum (if present) will be used to check that the payload files are present and accounted for, instead of re-calculating fixities and comparing them against the manifest. By default validate() will re-calculate fixities (fast=False). """


        self._validate_structure()
        self._validate_bagittxt()

        self._validate_fetch()

        self._validate_contents(processes=processes, fast=fast, completeness_only=completeness_only, callback=callback)

        return True