public final class PartialValue
A PartialValue contains instructions to modify another value, such as changing particular fields or
array elements, done in this way to save storage space. It is not meaningful by itself.
Apply this PartialValue onto an existing CompleteValue, yielding another CompleteValue.
Some AuditHistoryRows will have PartialValues, others will have CompleteValues. The first
row of a given value for a domain!key in a process context will always be a CompleteValue,
but subsequent rows may be PartialValues, which may then be applied _in order_ to the previous
CompleteValue to yield another CompleteValue.