Macros implement syntax and semantics. Arbitrary complex syntax. Check out the complex syntax for DEFGENERIC, DEFCLASS, DEFSTRUCT, defpackage ... LOOP

The syntax for DEFUN in Common Lisp:

  defun function-name lambda-list [[declaration* | documentation]] form*

The syntax for lambda-list then is:

  lambda-list::= (var* 
                  [&optional {var | (var [init-form [supplied-p-parameter]])}*] 
                  [&rest var] 
                  [&key {var | ({var | (keyword-name var)} [init-form [supplied-p-parameter]])}* [&allow-other-keys]] 
                  [&aux {var | (var [init-form])}*]) 

Lisp syntax is not atom | '(' atom* ')'. It's much more complex.

And so on.

Every Lisp has arbitrary complex syntax due to macros and special operators.