docs: def _vector_coef_op_left(func):
          """decorator for operator overloading when VectorCoef is on the
          left"""
          @wraps(func)
          def verif(self, vcoef):
              if isinstance(vcoef, VectorCoefs):
                  if len(vcoef.scoef1._vec) == len(vcoef.scoef1._vec):
                      return VectorCoefs(func(self, self.scoef1._vec,
                                                    vcoef.scoef1._vec),
                                         func(self, self.scoef2._vec,
                                                    vcoef.scoef2._vec),
                                          self.nmax,
                                          self.mmax)
                  else:
                      raise ValueError(err_msg['VC_sz_msmtch'] % \
                                      (self.nmax, self.mmax,
                                       vcoef.nmax, vcoef.mmax))
          
              elif isinstance(vcoef, numbers.Number):
                  return VectorCoefs(func(self, self.scoef1._vec, vcoef),
                                     func(self, self.scoef2._vec, vcoef),
                                     self.nmax,
                                     self.mmax)
              else:
                  raise TypeError(err_msg['no_combi_VC'])
          return verif