Extern num pattern tactic
This hint type is to extend Auto with tactics other than
 Apply and Unfold. For that, we must specify a
 cost, a pattern and a tactic to execute. Here is an example:
Hint discr : core := Extern 4 ~(?=?) Discriminate.
 Now, when the head of the goal is a disequality, Auto will
 try Discriminate if it does not succeed to solve the goal
 with hints with a cost less than 4.
 One can even use some sub-patterns of the pattern in the tactic
 script. A sub-pattern is a question mark followed by a number like
 ?1 or ?2. Here is an example:
Coq < Require EqDecide.
Coq < Require PolyList.
Coq < Hint eqdec1 : eqdec := Extern 5 {?1=?2}+{~ (?1=?2)} 
Coq <                                 Generalize ?1 ?2; Decide Equality.
Coq < 
Coq < Goal (a,b:(list nat*nat)){a=b}+{~a=b}.
1 subgoal
  
  ============================
   (a,b:(list nat*nat)){a=b}+{~a=b}
Coq < Info Auto with eqdec.
 == Intro a; Intro b; Generalize a b; Decide Equality; Generalize a0 p;
       Decide Equality.
    Generalize y0 n0; Decide Equality.
    
    Generalize y n; Decide Equality.
    
Subtree proved!