[Rd] Minimum of an ordered factor

[Thaler, Thorn, LAUSANNE, Applied Mathematics]

Kurt Hornik writes
> >> if (!all(sapply(args, is.ordered)) ||
> >> !all(sapply(level.list, identical, y = level.set))) {
> 
> I think it would be better to use something like
> 
>   ll <- lapply(args, levels)
> 
>   !all(sapply(ll, identical, ll[[1L]]))
> 
> [using union() is not quite right]

Yes definitely. This line is in fact just a relic from a previous idea I
had.
 
> The general comment is that if we support this I don't see why we
> should
> not also support c.ordered (and in fact also c.factor) with the same
> restrictions (identical level sequences for ordered and level sets for
> factors).  We already have Ops.factor and Ops.ordered using the same
> principle afaic.
> 
> If we add c.ordered, we should be able to encapsulate the identity of
> levels testing into this, and simply use
> 
>     x <- c(...)
> 
> and then call .Generic on the codes of x etc.

Sounds reasonable. Ack.

BR Thorn