Enrich code with comments and explanations

I had a really hard time understanding the provided code. The bison tutorial is not much of a help because it barely provides explanations.

Then I sat down and tried to read into the bison manual and tried to understand what happens.

So this is the outcome of me commenting on everything I did not understand. I would appreciate if you could read through it and tell my if I made any mistakes/ misunderstood some things.

flexBison/cplusplus/calc++.cc

@@ -6,7 +6,10 @@
 int
 main (int argc, char *argv[])
 {
+  // we build an interpreter, 
+  // hence we return an int as the result of the expressions we evaluate
   int res = 0;
+
   driver drv;
   for (int i = 1; i < argc; ++i)
     if (argv[i] == std::string ("-p"))
@@ -14,9 +17,11 @@ main (int argc, char *argv[])
     else if (argv[i] == std::string ("-s"))
       drv.trace_scanning = true;
     else if (!drv.parse (argv[i]))
+      // parse() returns 1 if it ran into an error
+      // so with !drv.parse we are here if everything was fine
       std::cout << drv.result << '\n';
     else
+      // error while parsing
       res = 1;
   return res;
 }
-