+val merge : t -> t -> t
+(** [merge a a'] creates a new automaton [a''] that evaluates both [a] and [a'']
+ in parallel
+*)
+
+val union : t -> t -> t
+(** [union a a'] creates a new automaton [a''] that selects node
+ selected by either [a] or [a']
+*)
+
+val inter : t -> t -> t
+(** [inter a a'] creates a new automaton [a''] that selects node
+ selected by both [a] and [a']
+*)
+
+val neg : t -> t
+(** [neg a] creates a new automaton [a'] that selects the nodes not
+ selected by [a]
+*)
+
+val diff : t -> t -> t
+(** [diff a a'] creates a new automaton [a''] that select nodes selected
+ by [a] but not selected by [a']
+*)
+