| Schedule of financial highlights |
| | Year Ended December 31, | | | | 2022 | | | 2021 | | | 2020 | | | 2019 | | | 2018 | | Per Share Data (1) | | | | | | | | | | | | | | | | Net asset value at beginning of period | | $ | 2.80 | | | | 2.44 | | | | 2.05 | | | | 2.30 | | | | 1.96 | | Net investment gain (loss) | | | 0.23 | | | | 0.27 | | | | 0.11 | | | | (0.14 | ) | | | (0.11 | ) | Net realized and unrealized gains | | | (0.11 | ) | | | 0.54 | | | | 0.41 | | | | 0.00 | | | | 0.45 | | Provision for income taxes | | | (0.03 | ) | | | (0.23 | ) | | | (0.05 | ) | | | 0.00 | | | | 0.00 | | Stock-based compensation | | | 0.05 | | | | 0.00 | | | | (0.02 | ) | | | 0.00 | | | | 0.00 | | Repurchase of common stock | | | 0.00 | | | | 0.00 | | | | 0.05 | | | | 0.00 | | | | 0.00 | | Other changes in equity | | | 0.03 | | | | 0.00 | | | | 0.00 | | | | 0.00 | | | | 0.00 | | Payment of common stock dividend | | | 0.00 | | | | (0.22 | ) | | | (0.11 | ) | | | (0.11 | ) | | | 0.00 | | Net asset value at end of period | | $ | 2.97 | | | | 2.80 | | | | 2.44 | | | | 2.05 | | | | 2.30 | | | | | | | | | | | | | | | | | | | | | | | Ratio / Supplemental Data | | | | | | | | | | | | | | | | | | | | | Per share market value of investments at end of period | | $ | 2.73 | | | | 2.95 | | | | 1.40 | | | | 0.36 | | | | 2.03 | | Shares outstanding at end of period | | | 6,185,255 | | | | 4,795,739 | | | | 4,793,739 | | | | 4,918,845 | | | | 4,918,845 | | Average weighted shares outstanding for the period | | | 5,333,028 | | | | 4,795,242 | | | | 4,830,691 | | | | 4,918,845 | | | | 4,918,845 | | Net assets at end of period | | $ | 18,342,339 | | | | 13,414,049 | | | | 11,640,887 | | | | 10,068,533 | | | | 11,278,889 | | Average net assets (2) | | $ | 15,733,550 | | | | 13,155,207 | | | | 10,504,563 | | | | 11,473,535 | | | | 10,341,702 | | Total investment return | | | 3.57 | % | | | 24.07 | % | | | 23.08 | % | | | (5.88 | )% | | | 17.24 | % | Portfolio turnover rate (3) | | | 128.80 | % | | | 168.67 | % | | | 61.11 | % | | | 7.63 | % | | | 26.93 | % | Ratio of operating expenses to average net assets (3) | | | (17.53 | )% | | | (10.30 | )% | | | (7.16 | )% | | | (7.27 | )% | | | (6.59 | )% | Ratio of net investment income (loss) to average net assets (3) | | | 9.16 | % | | | 9.89 | % | | | 5.35 | % | | | (5.86 | )% | | | (5.13 | )% |
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