def di(i, a, dt, tau):
    """
    Computes the change of concentration given an initial concentration `i`,
    a time increment `dt` and a constant `k`.

    Args:
        a (float): the initial concentration value
                   of the activator
        i (float): the initial concentration value
                   of the inhibitor
        dt (float): the time increment
        tau (float): the constant tau

    Returns:
        (float): the change of concentration
    """
    return dt/tau * (a - i)