input: Write a python function to extract the user name from a user ID by splitting the ID on the substring 'che'.
  assert extract_user_name('Johnche123')=='John'
  assert extract_user_name('Aliceche456')=='Alice'
  assert extract_user_name('Bobche789')=='Bob'
output: def extract_user_name(user_id):
      return user_id.split('che')[0]