Hi,  I am reading SICP and i am at the start of chap-3.  I am using PLT-Scheme ver-4.0 .I ran into problem with  mutable pairs. it was giving error. So instead of using  set-car! i used set-mcar!, but then even to create list  i have to create mutable list.  I have an older version also(ver-3.71). Is it a bad idea to  install older version or is there any other way out?  My main purpose is to do read SICP and do its exercise.  One more question, is there any other issues with  PLT-Scheme in later chapters of SICP?  Please help. -- Thanks and Regards Vinay Sachdev