In Common Lisp, DEFUN runs side effect code at compile time. For example it notes the function in the compile-time environment, it may record its definition/the

In Common Lisp, DEFUN runs side effect code at compile time. For example it notes the function in the compile-time environment, it may record its definition/the location of the definition, ... It also may rewrite the code it defines: for example by adding some declarations, etc.