input: Write a python function to find pairs in a list that sum up to a target value.
  assert search([2, 4, 3, 5, 7, 8], 7) == {(3, 4), (2, 5)}
  assert search([1, 2, 3, 4, 5], 5) == {(1, 4), (2, 3)}
  assert search([10, 20, 30, 40, 50], 60) == {(10, 50), (20, 40)}
  assert search([1, 1, 1, 1], 2) == {(1, 1)}
output: def search(lst, target):
      pairs = set()
      seen = set()
      for number in lst:
          complement = target - number
          if complement in seen:
              pairs.add((min(number, complement), max(number, complement)))
          seen.add(number)
      return pairs