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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 nelson_aalen_estimator(event, time):
 """Nelson-Aalen estimator of cumulative hazard function. Parameters event : array-like, shape = (n_samples,) Contains binary event indicators. time : array-like, shape = (n_samples,) Contains event/censoring times. Returns ------- time : array, shape = (n_times,) Unique times. cum_hazard : array, shape = (n_times,) Cumulative hazard at each unique time point. References .. [1] Nelson, W., "Theory and applications of hazard plotting for censored failure data", Technometrics, vol. 14, pp. 945-965, 1972. .. [2] Aalen, O. O., "Nonparametric inference for a family of counting processes", Annals of Statistics, vol. 6, pp. 701–726, 1978. """

    event, time = check_y_survival(event, time)
    check_consistent_length(event, time)
    uniq_times, n_events, n_at_risk = _compute_counts(event, time)

    y = numpy.cumsum(n_events / n_at_risk)

    return uniq_times, y