Add the given path to the decided place in sys.path

def add(self, path):
        """Add the given path to the decided place in sys.path"""
        
        # sys.path always has absolute paths.
        path = os.path.abspath(path)
        
        # It must exist.
        if not os.path.exists(path):
            return
        
        # It must not already be in sys.path.
        if path in sys.path:
            return

        if self.index is not None:
            sys.path.insert(self.index, path)
            self.index += 1
        else:
            sys.path.append(path)