I had inadvertently disallowed it.

The previous implementations were incorrect for large N.

This probably isn't a good idea, but Emitter.initPorts depends on it
to create the literal 0.

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.