PROC mark (INT CONST x, REAL CONST y):
  move (margin + sx * x, round ((y + 0.1) * sy));
  plot pixel
ENDPROC mark;

program:
  LET number of invisible steps = 40,
	  number of visible steps = 40,
	  number of points = 50;
  # On a faster computer choose larger values for better approximation #
  enter graphics mode;
  LET margin = 20;
  INT CONST sx :: (graphics x limit - margin) DIV number of points;
  REAL CONST sy :: real (graphics y limit) / 2.0;
  choose interval;
  UPTO number of points
  REP
    next point;
    some invisible steps;
    some visible steps
  ENDREP;
  wait for confirmation (1, graphics y limit - line height);
  leave graphics mode.

  choose interval:
    put ("Verhulst dynamics of rapidly growing population");
    line;
    put ("For background read The Beauty of Fractals");
    line;
    put ("by Peitgen and Richter, Springer Verlag 1986");
    LET limit = 3.0;
    REAL VAR x :: 1.9;
	INT VAR xpos :: 0;
    REAL CONST dx :: (limit - x) / real (number of points).
  
  next point:
	xpos INCR 1;
    x INCR dx.
  
  some invisible steps:
    REAL VAR y :: 0.1;
    UPTO number of invisible steps
    REP step
    ENDREP.
  
    step:
      y INCR x * (1.0 - y) * y.
    
  some visible steps:
    UPTO number of visible steps
    REP
      mark (xpos, y);
      step
    ENDREP.