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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 get_field(field):
 """Return a field object based on a dot-delimited app.model.field name"""

    if isinstance(field, djmodels.fields.Field):
        return field
    elif isinstance(field, basestring):
        field = field.split('.')
        if len(field) == 3:
            model = get_model(app=field[0], model=field[1])
        elif len(field) == 2:
            model = get_model(app=DEFAULT_APP, model=field[0])
        else:
            return None
            raise NotImplementedError("Unknown default model name. Don't know where to look for field %s" % '.'.join(field)) 
        field = model._meta.get_field(field[-1])
    return field