Comment by up2isomorphism

I want to point out the large part of "unreadability" feeling here comes from the fact the code is pre-ANSI C89. so tr(r,d)I d;{I z=1;DO(r,z=zd[i]);R z;} is just a function definition.

I would say the structurally this extremely easy to read considering there is zero comments and the way it is presented. One can make this readability obvious by just expand the macros, change indentations and add line breaks, or, you can just spend several days to "get used" to it like Roger Hui (and Arthur who just use this kind of style for life).