def feuler(f, t, y, dt):
    """
    Forward-Euler simplest possible ODE solver - 1 step.

    :param f:  function defining ODE dy/dt = f(t,y), two arguments
    :param t:  time
    :param y:  solution at t
    :param dt: step size
    :return: approximation of y at t+dt.
    """
    return y + dt * f(t, y)