diff -r f211d9cb856e -r 4d300409f2fe progs/mandelbrot.scala
--- a/progs/mandelbrot.scala	Fri Jun 22 13:34:05 2018 +0100
+++ b/progs/mandelbrot.scala	Tue Jun 26 01:49:32 2018 +0100
@@ -1,3 +1,6 @@
+// Mandelbrot pictures
+//   see https://en.wikipedia.org/wiki/Mandelbrot_set
+
 import java.awt.Color
 import java.awt.Dimension
 import java.awt.Graphics
@@ -19,9 +22,10 @@
   def abs() = Math.sqrt(this.re * this.re + this.im * this.im)
 }
 
+// to allow the notation n + m * i
 object i extends Complex(0, 1)
+implicit def double2complex(re: Double) = Complex(re, 0)
 
-implicit def double2complex(re: Double): Complex = Complex(re, 0)
 
 // some customn colours for the "sliding effect"
 val colours = List(
@@ -84,6 +88,8 @@
 }
 
 // main function 
+//    start and end are the upper-left and lower right corners 
+//    max is the number of maximum iterations
 def mandelbrot(start: Complex, end: Complex, max: Int) : Unit = {
   viewer.clearCanvas(black)
   
@@ -91,8 +97,8 @@
   val d_x = (end.re - start.re) / W
   val d_y = (end.im - start.im) / H
    
-  for (y <- (0 until H).par) {
-    for (x <- (0 until W).par) {
+  for (y <- (0 until H)) {
+    for (x <- (0 until W)) {
     
      val c = start + 
       (x * d_x + y * d_y * i)
@@ -107,6 +113,7 @@
   }   
 }
 
+
 // Examples
 //==========