The procedure calls Drestriction, which uses w if specified.
The algorithm used appears in the paper 'Polynomial and rational solutions of holonomic systems' by Oaku-Takayama-Tsai (2000). The method is to combine isomorphisms of Bjork and Kashiwara with the restriction algorithm.
i1 : W = QQ[x, D, WeylAlgebra=>{x=>D}] o1 = W o1 : PolynomialRing, 1 differential variables |
i2 : M = W^1/ideal(x*(D-1)) o2 = cokernel | xD-x | 1 o2 : W-module, quotient of W |
i3 : N = W^1/ideal((D-1)^2) o3 = cokernel | D2-2D+1 | 1 o3 : W-module, quotient of W |
i4 : DExt(M,N) 2 o4 = HashTable{0 => QQ } 2 1 => QQ o4 : HashTable |
The object DExt is a method function with options.