static <A extends Concept> A findConceptWhere(Class<A> c, O... params) {
  ret findConceptWhere(mainConcepts, c, params);
}

static <A extends Concept> A findConceptWhere(Concepts concepts, Class<A> c, O... params) {
  params = expandParams(c, params);
  
  // indexed
  if (concepts.fieldIndices != null)
    for (int i = 0; i < l(params); i += 2) {
      IFieldIndex<A, O> index = concepts.getFieldIndex(c, (S) params[i]);
      if (index != null) {
        for (A x : index.getAll(params[i+1]))
          if (checkConceptFields(x, params)) ret x;
        ret;
      }
    }
    
  // table scan
  for (A x : concepts.list(c)) if (checkConceptFields(x, params)) ret x;
  null;
}

static Concept findConceptWhere(Concepts concepts, S c, O... params) {
  for (Concept x : concepts.list(c)) if (checkConceptFields(x, params)) ret x;
  null;
}