towardsxy  -- Operation, two inputs.
     This operation applies only to the display  turtle.   The  two  inputs
     must  be  numbers, which are the x and y coordinates of a point on the
     TV screen.  The output is a number which is the heading to  which  the
     turtle  must  be  set,  in  order to point towards that point from its
     current position.  Note: this operation does not actually move or turn
     the  turtle.   You  must  use it as the input to setheading if that is
     what you want.
