For current EAPIs, PMS specifies that the "A" variable should be exported. For very large values of the "A" variable, this causes execve to fail with an "[Errno 7] Argument list too long" error as discussed in bug 719202.
In order to prepare for future EAPIs where the "A" variable will not be exported, it would make sense for portage to delay export of the "A" variable until the last moment. In theory, and ebuild could use "export -n A" to suppress "[Errno 7] Argument list too long" errors when spawning subprocesses.
Patch posted for review: