fnbex(xbe,ybe) = bex0 + bexx*xbe + bexy*ybe
     &          + bexxx*xbe*xbe + bexyy*ybe*ybe + bexxy*xbe*ybe
      fnbey(xbe,ybe) = bey0 + beyx*xbe + beyy*ybe
     &          + beyxx*xbe*xbe + beyyy*ybe*ybe + beyxy*xbe*ybe
      fnbez(xbe,ybe) = bez0 + bezx*xbe + bezy*ybe
     &          + bezxx*xbe*xbe + bezyy*ybe*ybe + bezxy*xbe*ybe