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Code:

[statedef your helper]
anim = your white anim
...

[State 1415, fade in]
type = trans
trigger1 = time < T1
trans = addalpha
alpha = X*(time), 256-X*(time)

[State 1415, solid]
type = trans
trigger1 = time = [T1,T2]
trans = addalpha
alpha = 256,0

[State 1415, fade out]
type = trans
trigger1 = time > T2
trans = addalpha
alpha = figure it out yourself ! :P

From time=0 to time=T1, there's the fade in. You must set X in order for the alpha value to go from 0,256 to 256,0 in T1 ticks.
From T1 to T2, you have the solid state
From T2 to whatever, you apply the inverse formula to go back to alpha=0,256

[statedef 3053]
anim = 2999

[State 1415, fade in]
type = trans
trigger1 = time < 20
trans = addalpha
alpha = 12*(time), 256-12*(time)

[State 1415, solid]
type = trans
trigger1 = time = [20,30]
trans = addalpha
alpha = 256,0

[State 1415, fade out]
type = trans
trigger1 = time > 30
trans = addalpha
alpha = 12*(time-30), 256-12*-(time-30)

[state 1415, destroyself]
type = destroyself
trigger1 = time = 50