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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 as_slug_expression(attr):
 """ Converts the given instrumented string attribute into an SQL expression that can be used as a slug. Slugs are identifiers for members in a collection that can be used in an URL. We create slug columns by replacing non-URL characters with dashes and lower casing the result. We need this at the ORM level so that we can use the slug in a query expression. """

    slug_expr = sa_func.replace(attr, ' ', '-')
    slug_expr = sa_func.replace(slug_expr, '_', '-')
    slug_expr = sa_func.lower(slug_expr)
    return slug_expr