input: 
  def create_blockchain_simple(blockchain_transactions):
      """ Build a simple blockchain system in Python.
      >>> create_blockchain_simple([{"sender": "Alice", "receiver": "Bob", "amount": "50"}])
      [{"index": 0, "timestamp": "#PLACEHOLDER#", "data": {}, "previous_hash": "0", "hash": "#SHA256HASH#"}, {"index": 1, "timestamp": "#PLACEHOLDER#", "data": {"sender": "Alice", "receiver": "Bob", "amount": "50"}, "previous_hash": "#SHA256HASH#", "hash": "#SHA256HASH#"}]
      """
  
output:     blockchain = [{"index": 0, "timestamp": "#PLACEHOLDER#", "data": {}, "previous_hash": "0", "hash": "#SHA256HASH#"}]
      for i, transaction in enumerate(blockchain_transactions):
          block = {
              "index": i + 1,
              "timestamp": "#PLACEHOLDER#",
              "data": transaction,
              "previous_hash": blockchain[-1]["hash"],
              "hash": "#SHA256HASH#"
          }
          blockchain.append(block)
      return blockchain