Equation with sign function

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Hi all,

I have the following piece of code

f(x) = 2*unit_step(x)
u = function('u')(x)
eqn = diff(u,x) + u == f(x)
v = desolve(eqn, u,ics=[0,0])
solve(v,x)

which gives me [x == 0, sgn(x) == -1] as solutions to the last equation. Could you help me find a way for Sage not to stop at sgn(x) == -1?

Thanks