Quadratic matrix multiplication

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The quadratic product "%&%" is a convenient (and efficient) function for pre- and post-multiplying a matrix B by another matrix A and its transpose A', ie.. A*B*A'

Quadratic form has several uses (see Quadratic form (statistics)

A %&% B is equivalent to A %*% B %*% t(A)

Conformability is cA=rB=cB