Construct a non-leaf XDOC tree
with a preprocessor directive or a tree concatenation at the root.
This is for internal use of the XDOC constructor library.
Users of this library should use
the constructor && for concatenation
or constructors for specific directives, e.g. @def.
See also make-tree-tag.
Definitions and Theorems
(defun make-tree-dir/&& (dir/&& trees)
(declare (xargs :guard (and (keywordp dir/&&)
(cons dir/&& trees))
(implies (keywordp dir/&&)
(equal (treep (make-tree-dir/&& dir/&& trees))