Lisp is all about data structures. Data can be expressed so easily no matter how complex it is. Lisp coding is merely writing minimum code to handle data structures. Even code is data. So it's no problem to extend Lisp with new commands. That's precisely coding around data.
In Java or C# however you have a lot of libraries to handle data but you don't have such freedom of data expression. You have to write a lot of code to express and handle complex data.
Lisp is all about data structures. Data can be expressed so easily no matter how complex it is. Lisp coding is merely writing minimum code to handle data structures. Even code is data. So it's no problem to extend Lisp with new commands. That's precisely coding around data.
In Java or C# however you have a lot of libraries to handle data but you don't have such freedom of data expression. You have to write a lot of code to express and handle complex data.