For any two 1-morphisms f:A→A′f\colon A\to A^{\prime}, g:B→B′g\colon B\to B^{\prime}, a 2-isomorphism
which we define as the image of the identity 2-morphism on (A⊗g)(f⊗B′)(A\otimes g)(f\otimes B^{\prime}) under the isotopy-induced homomorphism:
f
g
Scott Morrison, Kevin Walker, Paul Wedrich
Original source: arXiv:1907.12194v5
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