input: Write a function to return a substring from a given string using slicing based on the provided start and end indices. If the start index is greater than or equal to the end index, return an empty string.
  
  assert slicing("This is a slicing test", 8, 15)=="slicing"
  assert slicing("This is a slicing test", 0, 4)=="This"
  assert slicing("This is a slicing test", 5, 7)=="is"
  assert slicing("This is a slicing test", 16, 21)=="test"
  assert slicing("This is a slicing test", 2, 2)==""
  assert slicing("This is a slicing test", 10, 8)==""
output: def slicing(s, start, end):
      if start >= end:
          return ""
      return s[start:end]