%   The DRAWING INTERRUPTABLE check box, when on, creates
%an INTERRUPT button over the Control Panel while the
%fractal is being generated.  It also updates the fractal
%image periodically.  Pressing the INTERRUPT button will
%cause the the fractal generation process to terminate, and
%the partial image generated up to that point becomes the
%final image.  Note that this interruption is not immediate,
%especially on slower computers.  When DRAWING INTERRUPTABLE
%is on, the fractal generation process is somewhat slower.
%The advantage is that the user may interrupt the process
%whenever the fractal image is sufficiently well-defined.
%   Once the fractal has been drawn, a CONTINUE button
%appears over the Control Panel.  Pressing this button clears
%the fractal image and returns the program to its function
%entering/editing mode with all function settings unchanged.
%     A Note on memory capacity and image updating in the
%  DRAWING INTERRUPTABLE mode: in the student version of
%  MATLAB, a variable can contain at most 999 entries.  This
%  is represented in the program "makefrac.m" on line 7 by
%  the variable "maxvar".  This is the length of the vectors
%  containing the (x,y) plotting data generated for drawing
%  the fractal.  Enough of these are calculated to complete
%  the desired number of iterations.  When the DRAWING
%  INTERRUPTABLE mode is on, the program plots each data set
%  as it is generated.  As a result, you may increase the
%  frequency of image updating in this mode by decreasing the
%  value of "maxvar" in the program.  Similarly, the image
%  updating feature may be made faster but less frequent by
%  increasing this value.  However, you SHOULD NOT set this
%  value higher than the maximum allowable variable size of
%  the MATLAB version you are using.
%     The progress bar is also updated every "maxvar"
%  iterations, so if (# of iterations)<(maxvar), the progress
%  bar will not move until after image is finished.  However,
%  the program does execute slightly faster when this
%  inequality holds.