>>608
y=0
ffx=2x+ff(f0-x)
x=f0/2
ff(f0/2)=f0+ff(f0/2)
f0=0
ffx=2x+ff(-x)
x=0
fy=fffy
x=fy
f(ffy-y)=2fy
0=f(fffy-fy)=2ffy
f(fx+y)=2x
x=0
fy=0
0=2x
NG