My task is to convert the following grammar to CNF: $S \to SS \mid (S) \mid \lambda$
after removing lambda productions: $S\to SS, S\to (S), S\to(), S\to S$
after removing unit productions: $S\to SS, S\to (S), S\to()$
I got upto this point. Do not know how this grammar will be in CNF coz I am not sure if using () makes the grammar illegal. Need help how to proceed and make this grammar in CNF?