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4. Fair Value Measurements (Details 2) - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Senior Notes due 2022 $ 27,326,918 $ 28,521,734
Level 1    
Senior Notes due 2022 0 0
Level 2    
Senior Notes due 2022 27,326,918 28,521,734
Level 3    
Senior Notes due 2022 $ 0 $ 0