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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 valid(self, instance, schema):
 """Validate schema."""

        try:
            jsonschema.validate(instance, schema)
        except jsonschema.exceptions.ValidationError as ex:
            self.stderr.write("    VALIDATION ERROR: {}".format(instance['name'] if 'name' in instance else ''))
            self.stderr.write("        path:       {}".format(ex.path))
            self.stderr.write("        message:    {}".format(ex.message))
            self.stderr.write("        validator:  {}".format(ex.validator))
            self.stderr.write("        val. value: {}".format(ex.validator_value))
            return False

        try:
            # Check that default values fit field schema.
            for field in ['input', 'output', 'schema']:
                for schema, _, path in iterate_schema({}, instance.get(field, {})):
                    if 'default' in schema:
                        validate_schema({schema['name']: schema['default']}, [schema])
        except ValidationError:
            self.stderr.write("    VALIDATION ERROR: {}".format(instance['name']))
            self.stderr.write("        Default value of field '{}' is not valid.". format(path))
            return False

        return True