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:
51st_state

Context:
On May 15, 2013, Resident Commissioner Pierluisi introduced H.R. 2000 to Congress to "set forth the process for Puerto Rico to be admitted as a state of the Union," asking for Congress to vote on ratifying Puerto Rico as the 51st state. On February 12, 2014, Senator Martin Heinrich introduced a bill in the US Senate. The bill would require a binding referendum to be held in Puerto Rico asking whether the territory wants to be admitted as a state. In the event of a yes vote, the president would be asked to submit legislation to Congress to admit Puerto Rico as a state.

Question:
What did Senator Heinrich's bill require?

a binding referendum to be held in Puerto Rico asking whether the territory wants to be admitted as a state.