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

Whether expr, written in x and a candidate's variable
uSub, takes two real values at two points of x, uSub and every other symbol held at one value each. Then it is no function of
uSub alone, and no rewriting that keeps its value takes the x out of
it. False where the evaluations are not worth it (see
IsWorthEvaluatingAtRealPoints(AngouriMath.Entity,System.Boolean)), wherever a real value cannot be had at
either point, and where the two agree.