Construct an XDOC tree for a code block directive @({...}).
Macro:
(defmacro @{} (&rest trees) (list '@{}-fn (cons 'list trees)))
Function:
(defun @{}-fn (trees) (declare (xargs :guard (tree-listp trees))) (make-tree-dir/&& :@{} trees))
Theorem:
(defthm stringp-of-@{} (equal (treep (@{}-fn trees)) (tree-listp trees)))