code: def lincomb(self, a, x1, b=None, x2=None):
          
          return self.space.lincomb(a, x1, b, x2, out=self)
docs: Implement ``self[:] = a * x1 + b * x2``.
  
          Parameters
          ----------
          a : element of ``space.field``
              Scalar to multiply ``x1`` with.
          x1 : `LinearSpaceElement`
              First space element in the linear combination.
          b : element of ``space.field``, optional
              Scalar to multiply ``x2`` with. Required if ``x2`` is
              provided.
          x2 : `LinearSpaceElement`, optional
              Second space element in the linear combination.
  
          See Also
          --------
          LinearSpace.lincomb