WHERE's clause in PostgreSQL

I'm working with PostgreSQL's code in a project named "Temporal Aspects in PostgreSQL". I need change the programming's code in WHERE's clause. Where is a source code of the WHERE's clause? Thank you for atention.

Well, I cannot tell you exactly where it is, but why would you want to do that?? It will take you to lex and yacc, and all those things. Instead, if you need something different than and ordianry WHERE clause, you could write your custom function for it...

Other than that, I guess the source files or some header files could have info on Postgres' SQL interpreter...