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Accounting policies - Revenue recognized and billed on services (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2022
Dec. 31, 2021
Accounting policies    
Revenue recognized to date on contracts in progress $ 8,073,714 $ 6,109,396
Amounts billed to date on contracts in progress (8,071,419) (6,370,855)
Retainage 246,815 237,542
Net contract asset 249,110 (23,917)
Contract assets 828,962 489,961
Contract liabilities (579,852) (513,878)
Net contract asset (liability) $ 249,110 $ (23,917)