Multiply matrices MATRIX-1 and MATRIX-2, storing into MATRIX-3 if supplied. If MATRIX-3 is not supplied, then a new (ART-Q type) array is returned, else MATRIX-3 must have exactly the right dimensions for holding the result of the multiplication. Both MATRIX-1 and MATRIX-2 must be either one- or two-diimensional. The first dimension of MATRIX-2 must equal the second dimension of MATRIX-1, unless MATRIX-1 is one-dimensional, when the first dimensions must match (thus allowing multiplications of the form VECTOR x MATRIX)