+
+ val is_first_child : t -> node -> bool
+ val is_next_sibling : t -> node -> bool
+ val prev_sibling : t -> node -> node
+ (** [prev_sibling t n] returns the previous_sibling of node [n] in tree [t].
+ Returns [nil] if [n] is the first child of a node.
+ *)
+ val parent_of_first : t -> node -> node
+ (** [parent_of_first t n] returns the parent of node [n] in tree [t] .
+ Returns [nil] if [n] isn't the first child of a node.
+ *)