I have been trying to solve Pumping lemma in Coq.
I was on the third subgoal, Mapp
.
Lemma pumping : forall T (re : reg_exp T) s,
s =~ re ->
pumping_constant re <= length s ->
exists s1 s2 s3,
s = s1 ++ s2 ++ s3 /\
s2 <> [] /\
length s1 + length s2 <= pumping_constant re /\
forall m, s1 ++ napp m s2 ++ s3 =~ re.
My proof on MApp
is as follow.
Proof.
intros T re s Hmatch.
induction Hmatch
as [ | x | s1 re1 s2 re2 Hmatch1 IH1 Hmatch2 IH2
| s1 re1 re2 Hmatch IH | re1 s2 re2 Hmatch IH
| re | s1 s2 re Hmatch1 IH1 Hmatch2 IH2 ].
- (* MEmpty -- omitted *)
- (* MChar -- omitted *)
- (* MApp *)
intros T re s Hmatch.
induction Hmatch
as [ | x | s1 re1 s2 re2 Hmatch1 IH1 Hmatch2 IH2
| s1 re1 re2 Hmatch IH | re1 s2 re2 Hmatch IH
| re | s1 s2 re Hmatch1 IH1 Hmatch2 IH2 ].
- (* MEmpty *)
simpl. intros contra. inversion contra.
- (* MChar *)
simpl. intros. inversion H. inversion H1.
- (* MApp *)
simpl. rewrite app_length. intros.
apply add_le_cases in H.
destruct H as [H|H].
+ (*case pumping_constant re1 <= length s1 ommitted*)
+ apply IH2 in H. destruct H as [ss1 [ss2 [ss3 [H1 [H2 [H3 H4]]]]]].
exists (s1++ss1), ss2, ss3. split.
* rewrite H1. rewrite <- app_assoc. reflexivity.
* split. apply H2.
split. rewrite app_length.
assert (Hc: length s1<pumping_constant re1 \/ length s1>=pumping_constant re1).
apply lt_ge_cases.
destruct Hc as [Hc|Hc].
apply le_S in Hc.
apply Sn_le_Sm__n_le_m in Hc.
rewrite <- add_assoc.
apply (Plus.plus_le_compat _ _ _ _ Hc).
apply H3.
(* stuck *)
I am now stuck on case Hc: length s1>=pumping_constant re1
Goal:
2 goals
T : Type
s1 : list T
re1 : reg_exp T
s2 : list T
re2 : reg_exp T
Hmatch1 : s1 =~ re1
Hmatch2 : s2 =~ re2
IH1 : pumping_constant re1 <= length s1 ->
exists s2 s3 s4 : list T,
s1 = s2 ++ s3 ++ s4 /\
s3 <> [ ] /\
length s2 + length s3 <= pumping_constant re1 /\
(forall m : nat, s2 ++ napp m s3 ++ s4 =~ re1)
IH2 : pumping_constant re2 <= length s2 ->
exists s1 s3 s4 : list T,
s2 = s1 ++ s3 ++ s4 /\
s3 <> [ ] /\
length s1 + length s3 <= pumping_constant re2 /\
(forall m : nat, s1 ++ napp m s3 ++ s4 =~ re2)
ss1, ss2, ss3 : list T
H1 : s2 = ss1 ++ ss2 ++ ss3
H2 : ss2 <> [ ]
H3 : length ss1 + length ss2 <= pumping_constant re2
H4 : forall m : nat, ss1 ++ napp m ss2 ++ ss3 =~ re2
Hc : length s1 >= pumping_constant re1
______________________________________(1/2)
length s1 + length ss1 + length ss2 <=
pumping_constant re1 + pumping_constant re2
I tried solving it with cases H: length s1>=pumping_constant -> re1 length s1=pumping_constant re1 \/ length s1>pumping_constatn re1
.
It got me somewhere but the right case is tough to crack. How should I proceed?
Intuitively (I didn't install the specific libraries), I would start with a case analysis on length s1 >= pumping_constant re1
s2
to the right of the third element of the decomposition of s1
.length s1 < pumping_constant re1
and s1 ++ s2
is long enough, then length s2 >= pumping_constant re2
, and you can apply IH2
, then append s1
to the left of the first element of the decomposition of s2
.(to be verified)
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