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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 reference_extra_field(self, field, index):
 """Return the value of the field in 'extra', otherwise ''."""

        ref_data = self.reference_data(index)
        extra_fields = ref_data["extra"].split("\n")
        field_id = field + ":"
        matched = next((x for x in extra_fields if x.startswith(field_id)), None)
        if matched:
            return matched.replace(field_id, "", 1).strip()
        else:
            return ""