REPLACE COLOUR                                                     <Alt>+R

  This function is used to replace pixels of a specific colour in a block
  with another colour. The colour of the pixel under the cursor when the
  operation is invoked is the colour to be replaced. The current text colour
  (plotting colour) is the colour used for the replacement.

  Use the arrow keys, <Home>, <End>, <PgUp> and <PgDn> to resize the box
  specifying the area that will be effected. <Tab> toggles between the top
  left and the bottom right corners when resizing. <Enter> completes the
  operation causing the replacement to take effect. <Esc> aborts the
  operation leaving the screen as it was.