Returns the half-vectorization (i.e. vech) of the given matrix.

The half-vectorization, vech(A), of a symmetric nxn matrix A

is the n(n+1)/2 x 1 column vector obtained by vectorizing only

the upper triangular part of A.

For instance:

(= (half-vectorize (matrix [[a b] [b d]])) (matrix [a b d]))

Examples:

(def A (matrix [[1 2] [2 4]]))

(half-vectorize A)

References:

http://en.wikipedia.org/wiki/Vectorization_(mathematics)

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