Construct an XDOC tree for a long KaTeK 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)))