GivensMatrix := proc(  v :: Vector  # a 2-dim vector
                     )
   local c, s, r, G;

   r := sqrt( v[1]^2 + v[2]^2 );
   if r = 0 then
      return <<1,0>|<0,1>>;
   else
      c, s := v[1]/r, v[2]/r;
      return <<c,-s>|<s,c>>;
   end if;

end proc;