Jensen;
"begin"
   "comment" program to integrate f(x,y) over triangle (0,0), (1,0), (1,1);
   "comment" using Jensen"s device -- David Ho;

   "real" x, y;

   "real" "procedure" int(a, b, x, f);
   "comment" integrates f(x) over range a to b;
   "value" a, b; "real" a, b, x, f;
   "begin"
      "real" "procedure" intint(a, b, fa, fb);
      "value" a, b, fa, fb;  "real" a, b, fa, fb;
      "begin"
         "real" c, fc, ff;
         x := c := (a + b) / 2;
         fc := f;
         ff := (fa + 4*fc + fb)/6;
         "comment" answer is good enough if simrule no better than trapezoidal;
         "if"  abs( fa + fb - ff*2 ) < 0.001
         "then"
            intint := ff * (b - a)
         "else"
            intint := 0.0;
      "end";
      "real" fa;
      x := a;   fa := f;  x := b;
      int := intint(a, b, fa, f);
   "end";

   "real" answer;

"comment"   writetext(30, {int(0, 1, x, int(0, x, y, x*x + y*y)){c}});
   answer := int(0, 1, x, x*x);
"comment"   output(30, answer);

"end"