IsAProductOfSymbolicLinearFactors​(AngouriMath.​Entity,​AngouriMath.​Entity.​Variable,​System.​Boolean)

A quadratic beside the linears is allowed since the decomposition takes the linear
blocks by their Taylor coefficients and the quadratic's numerator in the ring
modulo the quadratic; with the gate asking for linears only,
u^4 (A + B u)/((a + b u)^4 (1 + u^2)) -- every rational function of the
tangent with a power of a linear in it -- went to the Hermite reduction below,
which answered in a^63 b^10.
https://github.com/asc-community/AngouriMath/issues/718
The partial fractions ask it without a linear where a symbolic factor is repeated:
quadratics alone are split by their residues just the same, and the half-angle
tangent's 2 (1 - t^2)^6/((1 + t^2)^5 (a t^2 + 2 b t + a)^2), which is Rubi's
cos(x)^6/(a + b sin(x))^2, has none. Where only a rational factor is repeated
the Hermite reduction answers shorter: sin(x)^2/(a + b cos(x)) by residues was
1864 characters for its 1109. The substitution search asks it with one, declining
only what the split surely takes: over two symbolic quadratics u = x^2 is often
the shorter answer.