This simplifies the handling of empty when statements.

Reimplement elsewhen in terms of otherwise.

Implement when/otherwise with a multi-command sequence, rather than a lone
but mutable command.  Now, all commands are immutable.  The command stack
is kaput.

Please, no more -1

I don't really like the syntax, but it's for backwards compatibility.

If we need anything from these files, we can re-add it later.