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Accounting policies - Revenue recognized and billed on services (Details) - USD ($)
 6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Accounting policies    
Revenue recognized to date on contracts in progress $ 6,434,715 $ 17,534,449
Amounts billed to date on contracts in progress (6,277,688) (17,791,928)
Retainage 244,025 312,130
Net contract asset 401,052 54,651
Contract assets 415,633 516,521
Contract liabilities (14,581) (461,870)
Net contract asset $ 401,052 $ 54,651