Fed sells government securities. Contraction is an expansion with a - sign. There is one bizarre feature that reserves temporarily go below requirements until a loan is paid off.
Contract: Sell G at whatever price necessary to get a bank to buy.
Bank 1: RR = 20%
Initial Position 10 G sale Loan paid off

Bank 2: RR = 20% First term geo series is D D in Bank 2
Initial Position 10 withdrawl to BK 1 Loan paid off

Second term geo series is D L in Bank 2 that becomes D D in Bank 3