input: Write a python function to load and preprocess customer data, including splitting the purchase history by ';'.
  assert preprocess_data('customers.csv').head().to_dict() == pd.DataFrame({
      'customer_id': [1, 2],
      'name': ['Alice', 'Bob'],
      'purchase_history': [['item1', 'item2'], ['item3']]
  }).head().to_dict()
  assert preprocess_data('another_customers.csv').shape[1] == 3
output: def preprocess_data(filename):
      import pandas as pd
      
      # Load the data
      df = pd.read_csv(filename)
      
      # Split the purchase history by ';'
      df['purchase_history'] = df['purchase_history'].apply(lambda x: x.split(';'))
      
      return df