Fix PAST decrease checks - #127
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This terminating countdown was rejected by the generated decrease check:
proc main(init_x: UInt) -> (x: UInt) {
x = init_x
@past(x + 1, 0.5, 1)
while 0 < x { x = x - 1 }
}
The precondition checked [0 < x] before x = init_x, so it could use x = 0 while init_x = 1.
Use [0 < init_x] in that precondition so the guard agrees with the state in which the loop iteration starts.
The countdown now verifies for every initial value; cover both this deterministic case and a probabilistic countdown.
Extract the exit bound, continuation bound, and expected decrease into separate helpers. Keep argument validation and orchestration in transform, with state initialization local to the decrease obligation. Name the generated procedures after their premises so verification output identifies which condition failed.
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The decrease check accepted
@past(1, 0.5, 1) while true { tick 1 }despite its infinite runtime.It used
proc, proving the inequality in the wrong direction; usecoprocto check that the invariant decreases by at least epsilon.The valid countdown
@past(x + 1, 0.5, 1) while 0 < x { x = x - 1 }was rejected because the generated precondition checked[0 < x]beforex = init_x.Use
[0 < init_x]so the guard and invariant refer to the same initial state.Fixes #122
Implementation assisted by Codex.