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I am trying to solve a differential equation with symbolic variables, but assume() does not seem to help answer maximas question:

t,a,b = var('t,a,b')
v = function('v',t)
de = diff(v,t)-a*v^2-b
assume((a*b)>0)
sol=desolve(de, dvar=v, ivar=t)

I still get the error message:

TypeError: ECL says: Maxima asks: Is  a*b  positive or negative?

How can i fix this?