Frama-c loop invariant - proof

Question

I just tried to prove a sort function in frama-c. However, when I proved the outer loop.
loop invariant 0 <= i <l;
loop invariant 0 < i < l ==> \forall int a,b; 0<=b <=l-i-1 <=a < l ==>
t[a]>=t[b];
There is always with the orange bullets. I refer to many examples and I cannot find the reason. Is there someone that can help me? Thanks!!
The following is my source code:
/*# predicate Swap{L1,L2}(int *a, integer l, integer i, integer j) =
\at(a[i],L1) == \at(a[j],L2) &&
\at(a[j],L1) == \at(a[i],L2) &&
\forall integer k; k != i && k != j
==> \at(a[k],L1) == \at(a[k],L2);
*/
/*# predicate Sorted{L}(int *a, integer l, integer h) =
\forall integer i,j; l <= i <= j < h ==> a[i] <= a[j] ;
*/
/*# requires \valid(t + (0..l-1));
requires 0 <= i < l;
requires 0 <= j < l;
assigns t[i],t[j];
ensures Swap{Old,Here}(t,l,i,j);
*/
void swap(int *t, int l, int i,int j){
int tmp;
tmp = t[i];
t[i] = t[j];
t[j] = tmp;
return;
}
/*# requires l >0;
requires \valid(t + (0..l-1));
ensures (\forall integer a; 0<=a <l
==> (\exists integer b; 0<= b < l
==> \at(t[b],Old)== \at(t[a],Here) ));
ensures Sorted{Here}(t, 0, l-1);
*/
void sort(int *t, int l) {
int i;
int j;
i=j=0;
/*# loop invariant 0 <= i <l;
loop invariant 0 < i < l ==> \forall int a,b; 0<=b <=l-i-1 <=a < l ==>
t[a]>=t[b];
*/
for (i=0;i<l;i++) {
/*#
loop invariant 0<= j < l;
loop invariant 0 < j < l ==>\forall int a; 0<= a <= j ==> t[a]<=t[j];
*/
for (j=0;j<l-1;j++) {
if (t[j] > t[j+1]){
swap(t,l ,j, j+1);}
}
}
}
and I use
frama-c-gui -wp sort.c