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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 requirement_is_installed(expr):
 """ Check whether a requirement is installed. :param expr: A requirement specification similar to those used in pip requirement files (a string). :returns: :data:`True` if the requirement is available (installed), :data:`False` otherwise. """

    required_dist = next(parse_requirements(expr))
    try:
        installed_dist = get_distribution(required_dist.key)
        return installed_dist in required_dist
    except DistributionNotFound:
        return False