code: def K(self,X,X2,target):
          
          AX = np.dot(X,self.transform)
          if X2 is None:
              X2 = X
              AX2 = AX
          else:
              AX2 = np.dot(X2, self.transform)
          self.k.K(X,X2,target)
          self.k.K(AX,X2,target)
          self.k.K(X,AX2,target)
          self.k.K(AX,AX2,target)
docs: Compute the covariance matrix between X and X2.