How to custom filter target objects?

Based on Mr. Lagues FieldOfView, I would like to know how to give custom statements dynamically to filter/remove some visibleTargets for example if the targets are in the same teams that can be determined by their teamscript.

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But I dont want to write these conditions in this script, since there could be plenty of those.

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How can I call a function from outer inside that loop that can do anything with col and returns a bool to add this as a valid target? I mean something like this:

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FieldOfView.cs:

 void Start(){
     StartCoroutine(FindTargetWithDelay(0.3f);
 }

 IEnumerator FindTargetWithDelay(float delay){
     while (true){
         yield return new WaitForSeconds(delay);
         FindVisibleTargets();
     }
 }

 IEnumerator FindVisibleTargets(float delay){
     yield return new WaitForSeconds(delay);

     visibleTargets.Clear();
     targetsInViewRadius = Physics.OverlapSphere(transform.position, viewRadius, targetMask).ToList();

     foreach(Collider col in targetsInViewRadius){
         if(CustomFilter(col)){ //This function should do the filtering (default true)
             Vector3 dirToTarget = (col.transform.position - transform.position);
             if(Vector3.Angle(transform.forward, dirToTarget.normalized) < viewAngle * 0.5f){
                 float dstToTarget = dirToTarget.magnitude;
                 if(!Physics.Raycast(transform.position, dirToTarget, dstToTarget, obstacleMask))
                     visibleTargets.Add(col.transform);
             }
         }
     }
 }

 //Override this function in other script. No filtering when no overriding reference is found
 public virtual bool CustomFilter(Collider col){
     return true;
 }

On an enemy with attached FieldOfView, i would call the overriding function:

Enemy.cs:

 public override bool CustomFilter(Collider col){
     return col.GetComponent<Team>().teamName != team.teamName;
 }

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But how do I pass this function to be called by FieldOfView? Transform.SendMessage is void and does not return a boolean.