function x = nul2pi( x )
% uhel_v_intervalu < 0 , 2*pi ) = nul2pi ( uhel_v_radianech )
for( i=1:size(x,1) )
  for( j=1:size(x,2) )
      
    while( x(i,j) >= 2*pi )
      x(i,j) = x(i,j) - 2*pi;
    end
    while( x(i,j) < 0 )
      x(i,j) = x(i,j) + 2*pi;
    end

  end
end