code: def _lincomb(self, a, x1, b, x2, out):
          
          _lincomb_impl(a, x1, b, x2, out)
docs: Implement the linear combination of ``x1`` and ``x2``.
  
          Compute ``out = a*x1 + b*x2`` using optimized
          BLAS routines if possible.
  
          This function is part of the subclassing API. Do not
          call it directly.
  
          Parameters
          ----------
          a, b : `TensorSpace.field` element
              Scalars to multiply ``x1`` and ``x2`` with.
          x1, x2 : `NumpyTensor`
              Summands in the linear combination.
          out : `NumpyTensor`
              Tensor to which the result is written.
  
          Examples
          --------
          >>> space = odl.rn(3)
          >>> x = space.element([0, 1, 1])
          >>> y = space.element([0, 0, 1])
          >>> out = space.element()
          >>> result = space.lincomb(1, x, 2, y, out)
          >>> result
          rn(3).element([ 0.,  1.,  3.])
          >>> result is out
          True