This warning occurred with the following code:

predicate ExistsSingleInstance(s : string32, delim : string32)
  {
    exists pos : nat ::
      (pos <= |s| - |delim| && forall x : nat :: x < |delim| ==> s[pos + x] == delim[x]) &&
      forall pos' : nat :: (pos' <= |s| - |delim| && forall y : nat :: y < |delim| ==> s[pos' + y] == delim[y]) ==> pos == pos'
  }

The verifier uses quantifications by finding good ways to instantiate them. More precisely, it uses forall quantifiers by instantiating them and it proves exists quantifiers by finding witnesses for them. The warning you’re getting says that nothing stands out to the verifier as a good way to figure out good instantiations.

In the case of pos, this stems from the fact that a good instantiation would be some pos for which the verifier already knows either pos <= … or forall x :: …, the former of which is not specific enough and the latter is too complicated for it to consider.

The “no trigger” warning is fixed by this refactoring:

predicate SingleInstance(s: string, delim: string, pos: nat)
{
  pos <= |s| - |delim| &&
  forall x : nat :: x < |delim| ==> s[pos + x] == delim[x]
}

predicate ExistsSingleInstance'(s: string, delim: string)
{
  exists pos : nat ::
    SingleInstance(s, delim, pos) &&
    forall pos' : nat :: SingleInstance(s, delim, pos') ==> pos == pos'
}

One more remark: The “no trigger” warning should be taken seriously, because it’s usually a sign that there will be problems with using the formula and/or problems with verification performance.