You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
European_Union_law

Context:
Following the Nice Treaty, there was an attempt to reform the constitutional law of the European Union and make it more transparent; this would have also produced a single constitutional document. However, as a result of the referendum in France and the referendum in the Netherlands, the 2004 Treaty establishing a Constitution for Europe never came into force. Instead, the Lisbon Treaty was enacted. Its substance was very similar to the proposed constitutional treaty, but it was formally an amending treaty, and – though it significantly altered the existing treaties – it did not completely replace them.

Question:
What type of treaty was the Lisbon Treaty?

an amending treaty