This function works with a DNA alignment (freshly) plotted on an
  interactive graphical device (i.e., not a file) with image.
  After calling alex, the user clicks twice defining a rectangle
  in the alignment, then this portion of the alignment is extacted and
  plotted on a new window. The user can click as many times on
  the alignment. The process is stopped by a right-click. If the user
  clicks twice outside the alignment, a message ``Try again!'' is
  printed.
Each time alex is called, the alignment is plotted on a new
  window without closing or deleting those possibly already plotted.
In all cases, the device where x is plotted is the active
  window after the operation. It should not be closed during the
  whole process.