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Parents at a high school near Dallas say the authors of a book on U.S. history misfired when they defined the Second Amendment -- and now one of the book's co-authors says the book is being revised.

The work book used at Guyer High School in the city of Denton, "The United States History: Preparing for the Advanced Placement Examination," includes the "summary" definition of the Second Amendment to include the right to "keep and bear arms in a state militia."

The edited definition is seen by gun-rights advocates as an affront to the Second Amendment, which states in full: "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed."

Parents at the school criticized the book for distorting the definition of the Second Amendment in favor of those opposed to gun rights.

John J. Newman, the book's co-author, told FoxNews.com that the text of the book was written before the 2008 Supreme Court ruling, District of Columbia v. Heller, which protects individuals' rights to possess firearms. He said the book's only intent was to summarize the Second Amendment and admitted that the current definition is incorrect. But he said a revised and corrected edition is set to be sold in May 2014.

"There was no devious intention in the definition," Newman said.

The book is a supplement to the actual text book, "American Pageant." The school says the text book gives the exact Bill of Rights.

"The teachers and the staff are aware of this "summary statement" and are teaching the amendments from the classroom textbook," a statement from the Denton School District said. "Please be assured that Denton ISD history teachers are disseminating the correct information on the Second Amendment."

Sean Getts, who has a daughter who is a junior at Guyer High School, told FoxNews.com she noticed the wording of the amendment and alerted him.

"I don't know if it was intentional or not, but clearly these authors interpreted the amendment in favor of gun control," he said. "I'm mainly concerned how this ends up in our school system."

Getts had posted the book's definition on Facebook and it went viral.

"I don't think the definition was put in the book in good faith," said Stephen Halbrook, a Second Amendment expert and author of "The Founders' Second Amendment."

"It's actually a total perversion of what the Second Amendment stands for."

He told FoxNews.com that the equivalent of this book's definition would be defining the First Amendment as: you have the right to free speech, as long as you speak in a state-sanctioned group.

Halbrook, who argued Second Amendment cases in front of the Supreme Court, said the book's authors could have edited the definition to: "A militia is necessary for a free state, and people have right to keep and bear arms."