a question about formatOf operations

[David Hough 754R work <754r@xxxxxxxxxxx>]

754R Draft 1.5.0 contains the language:

" 5.4.1 Arithmetic operations

 Implementations shall provide the following formatOf general-computational 
 operations, for destinations of
 all supported floating-point formats available for arithmetic, and, 
 for each destination format, for operands
 of all supported floating-point formats available for arithmetic with the same 
 radix as the destination format.

 formatOf-addition(source1, source2)
 The operation addition(x, y) computes x + y.  "

etc.


The intent is that for the listed operations, any combination of operand 
formats of a particular radix can be combined and rounded only once to 
any result format in that radix.

So in particular, if an implementation supports binary32, binary64,
and binary80 (or any other three binary formats), then it 
"provides" a way to compute

 binary64 = binary32 + binary80

with only one rounding error.    The most likely implementation approach,
from a hardware point of view, is to have operations with homogeneous
operands and narrowing results, e.g.

 binary32 = binary32 + binary32
 binary32 = binary64 + binary64
 binary32 = binary80 + binary80
 binary64 = binary64 + binary64
 binary64 = binary80 + binary80
 binary80 = binary80 + binary80

all with only one rounding error,
and then all combinations can be fabricated from these operations
and exact widening conversions.    That's expensive enough.

However my question is about programming language specification.
How exactly is one going to "provide" the most general forms of 
these operations in C or Fortran and other languages likely to support
multiple floating-point types?

Because "provide" might mean defining and implementing a function which
might be passed by reference, it would seem that all of the odd 
combinations like

 binary64 = binary32 + binary80 

might need to be given specific names and specific implementations 
and this is getting to be pretty expensive in documentation and
implementation.

I'm wondering if this is an issue that anybody has thought about enough to
plan solutions, and if the benefit is proportionate to the cost. 

[A similar issue arises with all the many kinds of conversions to all supported
integer types, required later in 5.4.1.]