grep -v 'mhshow-show-text/plain:' "$MH" >"$MH.new"
mv -f "$MH.new" "$MH"
cat >>"$MH" <<'EOF'
-mhshow-show-text/plain: echo `ap -format '%(void(lit %{charset}))%<(nonnull)-I %(putstr)%>' 0`
+mhshow-show-text/plain: charset="%{charset}"; echo ${charset:+-I $charset}
EOF
# check expansion of embedded %{charset} with no text following