def solver(f0,s0 , dt, t0 = 0.0, T = 10.0):
    s = [s0,]
    f = [f0,]
    t = [t0,]
    while(t[-1]<=T):
        f += [ f[-1] + dt*(2. - s[-1])*f[-1] ]
        s += [ s[-1] + dt*(f[-1] - 1.)*s[-1] ]
        t += [t[-1] +dt ]
    return s,f,t

if __name__ == '__main__':
    import sys
    from pylab import plot, show
    if len(sys.argv) != 4:
        sys.stderr.write("wrong num of params\n")
        sys.exit(2)
    
    s = solver(float(sys.argv[1]),float(sys.argv[2]),float(sys.argv[3]))
    
    plot(s[2],s[0])
    plot(s[2],s[1])
    plot(s[0],s[1])
    show()