Select Query Modification:Details needed

I am having a database course project where I need to make some modification in postgresql.

For this I need to modify  a select query and add some condition to it.

I need to find out the formclause and whereclause of the select query after it is parsed.To do that,I have found a structure ListCell,which contains a void * ptr_val.It seems that probably this ptr_val is holding some structure within which the strings are stored.But I am not being able to access that as I dont know what kind of structure is pointed by ptr_val.

I am trying to modify analyze.c ,method:transformSelectStmt(ParseState *pstate, SelectStmt *stmt)

Can anyone share any idea how to access the formclause and whereclause of the select statement after the query being parsed?

thanks,

Rajdeep