Unique complement of subspace

Hi,

**problem:**

Can it happen that a non-trivial subspace of a vector space V (i.e., a subspace different from 0 and V) has a unique complement?

attempt:

My answer to this would be no in general, but I do not know if there exists some exotic subspace for which this holds.

Here's my attempt at some sort of a proof:

Let K and H be subspaces of V.

Let be a basis of K.

Let be a basis of H.

K and H are complementary subspaces if and only if the complete set,

is a basis of V.

Say now that I have K and want to find a complementary subspace. Since every set of linearly independent vectors may be used as a basis, H is not unique.

Any suggestions are greatly appreciated.

Thanks