     
     internal func kernel(_ I: IntList) -> Matrix<R>? {
         guard let from = base[I], from.isFree,
-            let to = base[I + d.mDegree], to.isFree,
+            let to = base[I + dDegree], to.isFree,
             let A = dMatrix(I) else {
                 return nil // indeterminable.
         }

    
    internal func dSplits(_ I: IntList) -> Bool {
        guard let from = self[I],
            let to = self[I + d.mDegree],
            let A = dMatrix(I) else {
                return false
        }

    }
    
    public func describeMap(_ I: IntList) {
        print("\(I) \(self[I]?.description ?? "?") -> \(self[I + dDegree]?.description ?? "?")")
        if let A = dMatrix(I) {
            print(A.detailDescription)
        }
