Yes stay Away from them Authors, I recomend "Stephan K hays"
"Alex G mordine" "Masaaki Hatsumi" Which Who have been involved in Bujinkan ninjutsu And are Licensed Ninjutsu Instructers. Or 34th soke Grandmaster "Masaaki Hatsumi"
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"Bless be the Lord my strength Which Teacheth my hands to war and my fingers to Fight"