Syntax to make .altmacro feature %EXPR work?

[Erik Christiansen <dvalin at internode dot on dot net>]

Attempting to generate three symbols, fred0, fred1, fred2, is thwarted
by errors, despite various guesses at legal %EXPR syntax:

   .altmacro

   .macro foo hoo hah
   .equ fred%(\hoo) , \hah
   .endm


   i = 0 
   .rept 3
   foo i (42+i)
   i = i + 1
   .endr

gives, three times:     Error: expected comma after "fred"

The list file shows:     >> .equ fred%(i(42+i)),

Adding a separator:     .equ fred%(\hoo)& , \hah
gives a listing of:     >> .equ fred%(i(42+i))&,

Maybe %EXPR doesn't support macro parameter expansion? Let's try:

   .altmacro

   .macro foo n hah
   i = 0
   .rept \n
   .equ fred%(i)& , \hah&+i
   i = i + 1
   .endr
   .endm

   foo 3 42

The error message is still:
Error: expected comma after "fred"

and the listing is a much improved:        >> .equ fred%(i)&,42+i

But the % operator then appears to be running past the '&' separator, to
swallow all of the rest of the line.

Is there currently a way to elicit the documented behaviour with a
textually bounded expression?:

»
`Expression results as strings'
     You can write `%EXPR' to evaluate the expression EXPR and use the 
     result as a string.
«

The attempts were made both with the latest avr-as version on my recent
debian install: 2.26.20160125, and version 2.28 i686 native gas.


Erik