dear contract 0xsomething
do someAction( ) which changes your internal state
-- signed by 0xsomeone

doSomething()
start:
x = x + 1;
send 1 ETH to 0x999...
y = x + y;
end:

{x: 0, y: 0, balance: 10ETH}.

{x: 1, y: 1, balance: 9 ETH}