Question: DeterminingPDE Textbook Example

I am trying to repeat the 'first example' in Stephani & MacCullum, Differential equations see chapter 16 and (16.5).  The differential equation is,

pde := b1(x, y)*(diff(u(x, y), x))+b2(x, y)*(diff(u(x, y), y)) = 0

and I would like to compute DeterminingPDE.  The textbook answer is linear in b1 and b2 (as my 'by hand calculation' is) but Maple's,

DeterminingPDE(pde)

contains quadratic terms such as b1^2, b1*b2 etc.  I don't understand the appearance of the quadratic terms.  Is it possible for Maple to return an answer which is linear in b1 and b2?

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