What's the plan about language-joiner?

[Nala Ginrut]

Test code here:
------------------------cut--------------------------
# echo "(+ 1 1)" > aa.scm
# guild compile aa.scm --from=scheme --to=glil -o aa.glil
# guild compile aa.glil --from=glil --to=assembly -o aa.asm
------------------------end--------------------------

Error throws: "don't know how to join expressions"

And I checked the code, the problem is in "read-and-compile" in
module/system/base/compile.scm.
It tries to get "language-joiner", but only 'tree-il' has "joiner".
That means, if I try to compile any intermediate-language lower than
'tree-il', it'll throw the error message.

IMO, we ought to let guild compile any intermediate-language to any
lower level. And I can do such job follow these steps:
1. read out "aa.glil" 
2. parser-glil the glil code
3. (compile glil-code #:from 'glil #:to 'assembly) 

But I can't do it with 'guild compile' since it calls
"read-and-compile", which needs the weird "language-joiner". 
There's no any docs for 'language-joiner' in the manual. 
Here're the questions:
1. So what's the plan about 'language-joiner'? 
2. What's the utility?
3. Why only 'tree-il' has it?
4. What about implementing 'joiner' for other lower ILs?



Thanks!