human
Solve the following python programming problem given the following code:
def concat_string(node, stop = None) : 
    'Builds a string from a ast.BinOp chain.

    This will build a string from a series of ast.Str nodes wrapped in
    ast.BinOp nodes. Somthing like "a" + "b" + "c" or "a %s" % val etc.
    The provided node can be any participant in the BinOp chain.

    :param node: (ast.Str or ast.BinOp) The node to process
    :param stop: (ast.Str or ast.BinOp) Optional base node to stop at
    :returns: (Tuple) the root node of the expression, the string value
    ' 
    def _get(node, bits, stop = None) : 
        if (node != stop) : 
            bits.append ((_get (node.left, bits, stop) if isinstance (node.left, ast.BinOp) else node.left)) 
            bits.append ((_get (node.right, bits, stop) if isinstance (node.right, ast.BinOp) else node.right)) 
bits = [node] 
    while isinstance (node.parent, ast.BinOp) : 
        node = node.parent 
if isinstance (node, ast.BinOp) : 
        _get (node, bits, stop) 
return (stop, ' '.join ([x.s for x in bits if isinstance (x, ast.Str)])) 

    
    

    Fix the buggy line: return (stop, ' '.join ([x.s for x in bits if isinstance (x, ast.Str)]))
assistant
#Here is Python code to fix the buggy line: return (stop, ' '.

return (node, ' '.join ([x.s for x in bits if isinstance (x, ast.Str)]))