input: Write a Python function that converts a given positive integer into its Roman numeral representation in lowercase letters. The integer is constrained between 1 and 1000.
  assert int_to_roman_lower(19) == 'xix'
  assert int_to_roman_lower(152) == 'clii'
  assert int_to_roman_lower(251) == 'ccli'
  assert int_to_roman_lower(426) == 'cdxxvi'
  assert int_to_roman_lower(500) == 'd'
  assert int_to_roman_lower(1) == 'i'
  assert int_to_roman_lower(4) == 'iv'
  assert int_to_roman_lower(43) == 'xliii'
  assert int_to_roman_lower(90) == 'xc'
  assert int_to_roman_lower(94) == 'xciv'
  assert int_to_roman_lower(532) == 'dxxxii'
  assert int_to_roman_lower(900) == 'cm'
  assert int_to_roman_lower(994) == 'cmxciv'
  assert int_to_roman_lower(1000) == 'm'
output: def int_to_roman_lower(num):
      val = [
          1000, 900, 500, 400,
          100, 90, 50, 40,
          10, 9, 5, 4,
          1
      ]
      syb = [
          "m", "cm", "d", "cd",
          "c", "xc", "l", "xl",
          "x", "ix", "v", "iv",
          "i"
      ]
      roman_num = ''
      i = 0
      while num > 0:
          for _ in range(num // val[i]):
              roman_num += syb[i]
              num -= val[i]
          i += 1
      return roman_num