procedure main()
   every write( filter(even, ten) )
   #  every write( LoopingFilter(even, ten) )
end

procedure filter( p, g )
   every suspend(p(g()))
end

procedure LoopingFilter( p, g)
   local x
   while x:=g() do   # enters an infinite loop
     { write("x=", x); if p(x) then suspend(x)
     }
end

procedure even(n)
   return( ((n/2)*2) = n )
end

procedure ten()
   suspend(ints(10))
end

procedure ints(max)
   local i;
   i:=1;
   while i<=max do
   { suspend(i);
     i +:= 1
   }
end