input: Write a python function to sort a list of strings lexicographically, ignoring case.
  assert lexicographical_sort(["ruby", "go", "java", "python"]) == ["go", "java", "python", "ruby"]
  assert lexicographical_sort(["apple", "Banana", "cherry", "date"]) == ["apple", "Banana", "cherry", "date"]
  assert lexicographical_sort(["Zebra", "elephant", "Dog", "cat"]) == ["cat", "Dog", "elephant", "Zebra"]
output: def lexicographical_sort(strings):
      return sorted(strings, key=lambda s: s.lower())