Assume expression to be noninteger

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Hello! How to assume expression to be an integer? I try to do some symbolic integration while Sage asks to provide additional info about expression. MWE:

var('x a b')
assume(b > 0)
f = (exp((x-a)/b) + 1)**(-1)
(f*f).integrate(x, 0, oo)

gives:

ValueError: Computation failed since Maxima requested additional constraints; using the 'assume' command before evaluation may help (example of legal syntax is 'assume((2a)/b>0)', see assume? for more details) Is (2a)/b an integer?

How to assume that (2*a)/b is an arbitrary positive real number? Straightforward assume((2*a)/b, 'noninteger') gives:

TypeError: self (=2*a/b) must be a relational expression

Thanks!