Proportional hazards model.

    d: the dimension of the dataset
    timelines: the observational times
    constant: True for constant coefficients
    independent: the number of coffients to set to 0 (covariate is ind of survival), or
      a list of covariates to make indepent.

    returns a matrix (t,d+1) of coefficients

def constant_coefficients(d, timelines, constant=True, independent=0):
    """
    Proportional hazards model.

    d: the dimension of the dataset
    timelines: the observational times
    constant: True for constant coefficients
    independent: the number of coffients to set to 0 (covariate is ind of survival), or
      a list of covariates to make indepent.

    returns a matrix (t,d+1) of coefficients
    """
    return time_varying_coefficients(d, timelines, constant, independent=independent, randgen=random.normal)