The standard library uses a bool flag to indicate whether a splicing operation exhausted the source or filled the target, which is not very clear about what is being signaled. This enum replaces it.
FullSpan: This marks that a drain span has been completely filled with replacement bits, and any further replacement would require insertion rather than overwriting dead storage.EmptyInput: This marks that a replacement source has been run to completion, but dead bits remain in a drain span, and the dead range will need to be overwritten.