references:

• https://www.gnu.org/software/bison/manual/bison.html

• https://web.iitd.ac.in/~sumeet/flex__bison.pdf

  This book has some errors in it. Be careful!

flex manuaul: info flex

bison performs bottom-up/shift-reduce option while scanning input

debug

add

#define YYDEBUG 1
int yydebug = 1;

in definition section of the bison grammar file. Then,

• build with gcc -g flag.
• gdb breaks at yyparse function.

AST pictures