Structure constants for spin Hall--Littlewood functions

We provide a combinatorial formula for the structure constants of spin Hall–Littlewood functions. This is achieved by representing these functions and the structure constants as the partition function of a lattice model and applying the underlying Yang–Baxter equation. Our combinatorial expression is in terms of generalised honeycombs; the latter were introduced by Knutson and Tao for ordinary Littlewood–Richardson coefficients and applied to the computation of Hall polynomials by Zinn–Justin.