code: def get_matrix(self, x1, x2=None):
          
          x1 = self.parse_samples(x1)
          if x2 is None:
              return self.kernel.get_value(x1)
          x2 = self.parse_samples(x2)
          return self.kernel.get_value(x1, x2)
docs: Get the covariance matrix at a given set or two of independent
          coordinates.
  
          :param x1: ``(nsamples,)`` or ``(nsamples, ndim)``
              A list of samples.
  
          :param x2: ``(nsamples,)`` or ``(nsamples, ndim)`` (optional)
              A second list of samples. If this is given, the cross covariance
              matrix is computed. Otherwise, the auto-covariance is evaluated.