Shim Code for Modal Arguments

I've described a new parameter convention that I call an @arg which could be supported by the evaluator. But here is a bit of code to implement the trick in present-day Ren-C:

old-append: :append

append: function [
    series
    :nosplice [<skip> sym-word! sym-path! sym-block! sym-group!]
    value [<...> any-value!]
    /only
][
    switch type of :nosplice [
        null [old-append/(only) series take value]  ; no @... so act variadic
        sym-block! [old-append/only series as block! splicer]
        sym-group! [old-append/only series do as group! splicer]
        sym-word!
        sym-path! [old-append/only series get splicer]
   ]
]

Consequences

This leaves splicing the default:

>> append [a b c] [d e]
== [a b c d e]

But if you use an @... at the callsite it offers alternate ways of requesting /ONLY:

>> append [a b c] @[d e]
== [a b c [d e]]

The trickery though is that it has to be a literal @... at the callsite. Indirectly that doesn't apply (if it comes from a reference or expression), it will append in the typical fashion, whatever that would be:

>> item: @[d e]
== @[d e]

>> append [a b c] item
== [a b c d e]  ; or should only plain BLOCK! splice?

The callsite permits conventional expressions for the plain case (now an /ONLY behavior), written out as a full evaluation (e.g. the argument is not quoted):

>> append [a b c] reverse copy [d e]
== [a b c e d]

On the other hand, the /only cases are all going through a quoted argument...but these provide variations besides an @-block:

>> append [a b c] @item
== [a b c [d e]]

>> append [a b c] @(reverse copy item)
== [a b c [e d]]

And again, these variations are visible at the callsite; they don't accidentally sneak in to run code as /ONLY if your callsite didn't make it look that way just due to some indirect value:

>> item: @(reverse copy item)
== @(reverse copy item)

>> append [a b c] item
== [a b c reverse copy item]  ; ANY-ARRAY! splice?

If you really do want to deliberately append an @... then you can do so with quoting (which you'd have to do for a WORD! or other evaluative type that you didn't mean "do something evaluatory with")