code: def is_valid_country_abbrev(abbrev, case_sensitive=False):
      
      if case_sensitive:
          country_code = abbrev
      else:
          country_code = abbrev.upper()
  
      for code, full_name in COUNTRY_TUPLES:
          if country_code == code:
              return True
  
      return False
docs: Given a country code abbreviation, check to see if it matches the
      country table.
  
      abbrev: (str) Country code to evaluate.
      case_sensitive: (bool) When True, enforce case sensitivity.
  
      Returns True if valid, False if not.