Matrix decomposition: orthogonal complement

Hello,

How I can compute in R the orthogonal complement of  one matrix?

If  A (n x m ) matrix of full column rank (n>m), its orthogonal 
complement is denoted by A_ .

A_  is  n X (n-m) matrix of full column rank and such that A'A_=0.

I  need to compute A_.   How I can compute A_ in R?

Best Regards,
/Florin

Florin G. Maican
Ph.D. candidate, Department of Economics
School of Economics and Commercial Law
Gothenburg University, Sweden
E-mail: Florin.Maican at handels.gu.se
P.O.Box 640 SE-405 30 Gothenburg, Sweden 
Phone no +46 31 773 4866
Fax no +46 31 773 4154