In category theory there are lots of examples of isomorphisms that cannot be strictified to become identities. For instance, every monoidal category is equivalent to a strict monoidal category, where the associativity and unit isomorphisms are identities, but not every braided monoidal category is equivalent to a strictly braided one where the braiding is an identity.


In higher category theory, constraints are in general (adjoint) *equivalences* rather than isomorphisms. For instance, the associativity and unit 2-cell constraints for a tricategory (weak 3-category) are equivalences, but the 3-cell constraints (such as the interchanger) are still isomorphisms, since there is "no room" for anything weaker (there being no 4-cells in a tricategory). Every tricategory is equivalent, not to a strict 3-category where all constraints are identities, but to a Gray-category where the associators and unitors are identities but the interchanger is not.


I am looking for an example of a higher-categorical structure containing constraint *equivalences* which cannot be strictified even to become *isomorphisms* (not necessarily identities). Since the only nontrivial constraints in a Gray-category are top-dimensional and hence isomorphisms, the first place to look for this would be in some sort of 4-category. But we can also make it more manageable by being somewhat degenerate. A triply degenerate 4-category (exactly one 0-, 1-, and 2-cell) would be (by the delooping hypothesis) a symmetric monoidal category, with no room for any equivalences that aren't isomorphisms, so the next level of complexity seems the first place to look.


A doubly degenerate 4-category should be the same as a braided monoidal bicategory, and by the coherence theorem for tricategories, everyone of those is equivalent to a braided Gray-monoid (a Gray-monoid being a Gray-category with one object). Howver, the braiding in a braided Gray-monoid is, *a priori*, still only an *equivalence*, so one way to make this question precise would be:



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> Is every braided Gray-monoid equivalent to one whose braiding is an isomorphism, rather than merely an equivalence?
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Can equivalences be strictified to isomorphisms?
It appears that the answer to the corresponding question for *symmetric* Gray-monoids is yes; this was shown by [Schommer-Pries](https://arxiv.org/abs/1112.1000v2) and cleanly reformulated by [Gurski-Johnson-Osorno](https://arxiv.org/abs/1503.07824). Nick [says](https://golem.ph.utexas.edu/category/2018/01/the_stable_homotopy_hypothesis.html#c053431) that he thinks the braided case should work similarly.


