work opened up a possibility that the OAM appearing in the decomposition (II) may also be related to observables. Since the relation between the OAM appearing in the decomposition (I) and the observables is already known, this means that we may be able to isolate the correspondent of [*potential angular momentum*]{} term appearing in Feynman’s paradox as a difference between the two OAMs as $$\begin{aligned}
 L_{pot} \ = \ L_{mech} \ - \ L_{``can"}\end{aligned}$$ However, one must be careful about the presence of very delicate problem on the sum rules containing GTMDs and/or ordinary TMDs. (See, for example, the textbook [@BookCollins], which discusses the delicacies of TMDs in full detail.) Once quantum loop effects are taken into account, the very existence of TMDs satisfying gauge-invariance and factorization (universality or process independence) simultaneously is being questioned. Is process-independent extraction of $L_{``can"}$ really possible ? One must say that it is still a challenging open question.

What is “potential angular momentum" ?
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We have shown that the key quantity, which distinguishes the two physically different nucleon spin decompositions, is what-we-call the “potential angular momentum" term. To understand its physical meaning more throughly, and also to understand the reason why there exist 