def flatten(self, lst=None):
        """syntax.flatten(token_stream) - compile period tokens

        This turns a stream of tokens into p-code for the trivial
        stack machine that evaluates period expressions in in_period.
        """
        tree = []
        uops = []                       # accumulated unary operations
        s = Stack()
        group_len = 0                   # in current precendence group
        for item in lst:
            if type(item) == type([]):
                # Subexpression.
                tree = tree + self.flatten(item)
                group_len = group_len + 1
                # Unary ops dump, for things like: '!(Monday|Wednesday)'
                for uop in uops:
                    tree.append(uop)
                uops = []
            elif item in self.ops and item not in self.uops:
                # Operator.
                if not s.empty():
                    prev_op = s.pop()
                    # If the precendence of the previous operation is
                    # higher then dump out everything so far, ensuring the
                    # order of evaluation.
                    if _precedence[prev_op] > _precedence[item]:
                        s.push(prev_op)  # put it back
                        for i in range(group_len - 1):
                            tree.append(s.pop())
                        group_len = 0
                    else:
                        s.push(prev_op)
                    s.push(item)
                else:
                    s.push(item)
            elif item in self.uops:
                uops.append(item)
            else:
                # Token of some sort.
                tree.append(item)
                group_len = group_len + 1
                # Dump any unary operations.
                for uop in uops:
                    tree.append(uop)
                uops = []

        while not s.empty():
            tree.append(s.pop())

        # Drop any remaining unary operations.
        for uop in uops:
            tree.append(uop)

        return tree