Reasonably portable technique for local variables in shell scripts

local.sh

#!/bin/sh -e
# This shell script demonstrates how to use the local keyword to create
# dynamically-scoped variables in various shells.  Notably, this technique works
# with AT&T ksh, as well as bash, dash, mksh, pdksh, zsh, busybox sh, and other
# Debian Policy-compliant sh implementations.

# Simple Perl-compatible testing framework.  Produces TAP output.
COUNT=0

is () {
    COUNT=$(($COUNT + 1))
    if [ "$1" = "$2" ]
    then
        printf 'ok %d\n' $COUNT
    else
        printf 'not ok %d\n' $COUNT
    fi
}

finish () {
    printf '1..%d\n' $COUNT
}
trap finish EXIT

# True if this is AT&T ksh, but not MirBSD ksh or pdksh.
is_att_ksh () {
    [ -n "$KSH_VERSION" ] && [ -z "${KSH_VERSION##*Version AJM*}" ]
}

is_att_ksh && alias \local=typeset

# A helper to declare a function.  We declare our actual code in a function
# starting with an underscore (e.g. _f).  We then use this helper to create a
# wrapper (e.g. f) around that.  With AT&T ksh, this uses the ksh-style function
# syntax to create the scoping that's required for the local (typeset) to work
# properly.  The local variables are technically scoped to f, not _f, but that
# doesn't practically matter.
#
# For bash, dash, mksh, pdksh, busybox sh, and anything else, we use the
# standard POSIX syntax to call the underlying function.  This is more portable.
decl () {
    if is_att_ksh
    then
        eval "function $1 { _$1 \"\$@\"; }"
    else
        eval "$1 () { _$1 \"\$@\"; }"
    fi
}

# To actually declare a function, we first declare the actual function with an
# underscore in front of it…
_f () {
    local a
    a="$1"
    is "$a" 2
}

# …and then we use the decl helper to create the wrapper function.
decl f

# A simple test that the local keyword does indeed work in the shell in
# question.
a=1
is "$a" 1
f 2
is "$a" 1