input: 
  class VersionControl:
      def __init__(self, lines):
          self.lines = lines
          self.mark = 0
   
      def commit(self):
          self.mark += 1
          log_file = open("log_file.txt", "a+")
          log_file.write(str(self.mark) + ": " + str(self.lines))
          log_file.close()
   
      def restore(self):
          log_file = open("log_file.txt", "r")
          lines = log_file.readlines()
          to_mark = self.mark-1
   
          self.lines = lines[to_mark]
          log_file.close()
  
  # Example:
  # version_control = VersionControl(["line1", "line2"])
  # version_control.commit()
  # version_control.lines = ["line1_changed", "line2"]
  # version_control.commit()
  # version_control.restore()
  # assert version_control.lines == ["line1", "line2"]
  
output:     
          log_file = open("log_file.txt", "r")
          lines = log_file.readlines()
          to_mark = self.mark-1
          
          if to_mark >= 0 and to_mark < len(lines):
              self.lines = eval(lines[to_mark].split(": ")[1])
          log_file.close()
  