code: def add_child_hash(self, child_hash):
          
  
          # Hash must generate binary keys
          if not (isinstance(child_hash,PCABinaryProjections) or isinstance(child_hash,RandomBinaryProjections) or isinstance(child_hash,RandomBinaryProjectionTree)):
              raise ValueError('Child hashes must generate binary keys')
  
          # Add both hash and config to array of child hashes. Also we are going to
          # accumulate used bucket keys for every hash in order to build the permuted index
          self.child_hashes.append(child_hash)
docs: Adds specified child hash.
  
          The hash must be one of the binary types.