sed stop insert the line after the first occurrence of the pattern

Lets take such text file named "/tmp/test":

Code:yyyy-1
xxxx-2
xxxx-3
zzzz-4
xxxx-5I want to insert additional line containing "aaaa-6" before first occurrence of line containing string "xxxx" at the beginning of line. But only one time, as I wrote at first occurrence.

For this purpose I want to use the script like this:

Code:#!/bin/sh

entry="aaaa-6"
sed -i "/^xxxx*/a $entry" /tmp/testBut this script insert after each occurrence of pattern.
Any idea how to stop sed after first occurrence?
Notice that /bin/sh is used (busybox) not bash.