Thanks for the revision.   I'm going to implement it in Gauche
and try it out.

On 'nfx': I think your concern about users relying on
implementation-dependent extention inadvertently is reasonable.
My $.02 is that you just say "if an implementations claims it's
srfi-105, leave nfx unbound; if an implementation wants to put some
default meaning on nfx, call it something other than srfi-105."

Of course a srfi-105 implementation may supply a library that binds
nfx and let users load the library explicitly if they desire.

--shiro