namespace lasd {
template <typename Data>
void AuxFoldExists(const Data& dat, const void* val, void* exists){
if(dat == *((Data*)val)){
*((bool*)exists) = true;
}
bool FoldableContainer<Data>::Exists(const Data& dat) const noexcept{
bool exists = false;
FoldPreOrder(&AuxFoldExists<Data>, &dat, &exists);
return exists;