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