input: Write a function to generate the first n numbers in the Fibonacci sequence.
  assert fibonacci(5) == [0, 1, 1, 2, 3]
  assert fibonacci(10) == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
  assert fibonacci(15) == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377]
output: def fibonacci(n):
      if n <= 0:
          return []
      sequence = [0, 1]
      while len(sequence) < n:
          sequence.append(sequence[-1] + sequence[-2])
      return sequence[:n]