KeepsTheVariableThroughEveryRewriting​(AngouriMath.​Entity,​AngouriMath.​Entity,​AngouriMath.​Entity.​Variable,​AngouriMath.​Entity.​Variable,​AngouriMath.​Entity,​AngouriMath.​Entity)

Whether the quotient of an integrand by a candidate's derivative, written in the
candidate's name uSub, keeps x through every
rewriting SolveBySubstitution(AngouriMath.Entity,AngouriMath.Entity.Variable,System.Boolean) tries after it. It takes two values at two
points of x where the name is held, and a simplification keeps a value; and the steps
that write what is left of x in the candidate do not apply: the candidate is no
polynomial of a degree past one, no special function, no sine or cosine, the quotient is
no constant times its power from -2 to 2 at three points, and no factor of the integrand
is a constant times its derivative. False wherever a value cannot be had.