I wrote a fair amount of code for a startup, but I haven't signed a contract before doing so. The only document that I signed with them does not mention the fact that I have to pass the rights on the code to them, and after a consulting with a lawyer it seems that I own the full rights. Now I want to preemptively correct this situation by giving them some sort of exclusive license. Is there an existing license for closed-source, exclusive use that is used in these cases or I simply write somewhere "I grant exclusive license to use and modify this piece of code to FooBar-inc at the followings conditions: bla bla bla signed me, them"?