Middle.Mem_patternThis type represents whether or not an autodiff type can be represented as an Array of Structs (AoS) or as a Struct of Arrays. This applies to matrices, vectors, row vectors, and arrays of those types.
In the C++ this allows us to swap out matrix types from an Eigen::Matrix<stan::math::var_value<double>, Rows, Cols> to an stan::math::var_value<Eigen::Matrix<double, Rows, Cols>>.
val sexp_of_t : t -> Sexplib0.Sexp.tval pp : Stdlib.Format.formatter -> t -> unit