TOGGLE BORDER FORMAT                                               <Alt>+K

  When you are over one of the corner line characters (+,+,+,+, etc.), this
  function will toggle the line format between single lines, double lines,
  and the two single/double combinations. This doesn't only effect the
  character beneath the cursor, but also any lines connected to it, so if
  you have drawn a fancy border around something, the whole border will be
  altered.