input: Write a function to sort a list of strings lexicographically by their first letter.
  assert sortLexicographical(["red", "blue", "green", "purple", "pink", "black", "white"]) == ['black', 'blue', 'green', 'pink', 'purple', 'red', 'white']
  assert sortLexicographical(["apple", "banana", "cherry", "date", "fig", "grape"]) == ['apple', 'banana', 'cherry', 'date', 'fig', 'grape']
  assert sortLexicographical(["zebra", "yak", "xenon", "wolf", "vulture", "urchin"]) == ['urchin', 'vulture', 'wolf', 'xenon', 'yak', 'zebra']
output: def sortLexicographical(strings):
      return sorted(strings, key=lambda s: s[0])