       10 // CLOSED sample program
       20 //
       30 a:=1
       40 aap
       50 SYS assert, a=2
       60 //
       70 PRINT "Ok"
       80 //
       90 PROC aap CLOSED
      100   a:=9
      110   olifant
      120 ENDPROC aap
      130 //
      140 PROC olifant  // Not CLOSED == OPEN!
      150   SYS assert, a=1
      160   a:=2
      170 ENDPROC olifant
