GHC Lexer

Hello, to add a new keyword, say "struct" to the GHC, I have added ITstruct to data type Token in module Lex.lhs. Further on I have adapted ghcExtensionKeywordsFM. Is this already sufficient for lexing? In LexCore there is a function lexKeyword without a comment. It is called by lexer if the first character is %. What does this mean? Thank you, Steffen