SIA[(o,t),(s',t')] fj + If(O,t') f(O,t)I

 SVar[( ,t),( )](f) + If(Ot') f( ,t) .

2) If(s',t) f(s,t) = If(s',t) f(s,t) f(s',O) + f(s,O)

 + f(s',0) f(s,O)l

 Sjf(s',t) f(s,t) f(s',O) + f(s,O)|

 + |f(s',O) f(s,0)I

 SI[(s,),(s't)](f)l + If(s',0) f(s,3o)

 SVar[(s,0),(s t)](f) + jf(s',0) f(s,0) .

Putting everything together, we have

 If(s',t') f(s,t)l If(s',t') f(s',t)j + ~ f(s',t) f(s,t)

 SVar[(ot),(s ,t )] (f) + If(Ot') f(Ot)I

 + Var[(sO),(s.'t)](f) + If(s'O) f(s0)

 [Varf(O,),(s',t')](f) Var [( ,),( ,t)]

 + If(0,t') f(0,t)I + jf(s',0) f(s,0)D

(of. Figure 2-9). As we showed above, each of the three terms on the


Figure 2-9 Rectangles used in (1) and (2)