form index
what it holds
The form index holds which body the record currently wears. It is a single integer and it points at one entry in the prewritten ladder.
Because it moves only on advance, it is exactly equal to the number of bands crossed.1 That equality is an invariant and is checked on the math page.
The body it points at is drawn from the record state rather than stored as an image, so the same index produces the same figure for every observer.
The index is the whole of what the field holds. Everything visible about the body is computed elsewhere from it and from the trait bits.
what writes it
Advance raises it by one. The instruction requires that witnessed has crossed the next threshold, and where that condition does not hold the call does nothing.
The increment is always one. There is no path that crosses two bands in a single call, even where witnessed has passed two thresholds since the last advance.
The same call rerolls the trait bits.1 The index and the roll therefore always move together and never separately.
No other instruction writes the field. Mark, lapse and sweep leave it untouched whatever they do to the totals.
what it cannot do
The index is not chosen. No caller selects which body appears, and the call that raises it takes no argument that could express a preference.
It is not purchasable. Nothing in the contract accepts payment in exchange for movement on the ladder.
It is not reversible. There is no instruction that lowers it and no condition under which it returns to a previous value.
It also cannot exceed the last band, so at the top of the ladder the field stops moving while witnessed continues.
how to read it
The form index tells you how much witnessed time the record has accumulated, expressed as a rung rather than a number.
It is coarse by design. Two records at the same index may hold witnessed totals far apart, since the index only reports the last threshold passed.1
A stationary index with a rising witnessed means the next threshold has not yet been reached or advance has not yet been called.
The table lists each form with the band that opened it and how long the index has held there.
| index | band that opened it | held since | advances |
|---|---|---|---|