The whole program trusts definitions in mac.h [1] like:

    #define IF if(
    #define THEN ){
    #define ELSE } else {
    #define ELIF } else if (
    #define FI ;}

    #define BEGIN {
    #define END }
    #define SWITCH switch(
    #define IN ){
    #define ENDSW }
    #define FOR for(
    #define WHILE while(
    ...

Isn't it nowadays considered bad practice? After taking a glance at the code, I see there might be some advantages like not forgetting to add missing {}. Is there any other explanation on why they created a dialect on the top of C using the preprocessor?

[1] https://www.tuhs.org/cgi-bin/utree.pl?file=V7/usr/src/cmd/sh...

EDIT: fix English