Help needed to ID plant

in Plants
Can anyone tell me what this is?
Is is friend or foe? I didn't buy it, but might have been given it. I also found some growing out of my path today a lot further up the garden. It might be good in for a container display for a bit of height at the back. But if it's a self-seeding, thuggish invader then I'll just get rid. Grateful for any ideas.
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Realised last pic was a bit small so here's a larger version, sorry it's on it's side.
Sorry, can't tell from pic. Need close-up. You may just have to wait and see what it does come Aug/Sep - traditionally when grassy things show their hand and flower.
DaisyHC, I can't enlarge it.
Is the stem round, cylindrical rather than flat ...as in a blade of grass?
It looks like a rush... possibly Juncus effusus.
http://www.plant-identification.co.uk/skye/juncaceae/juncus-effusus.htm
http://en.wikipedia.org/wiki/Juncus_effusus
Hmm that could be it. The stems are cylindrical. I'll chuck it then. Back to planting proper plants! Thanks very much Peetey