The reduction operators come from APL (A Programming Language) and `reduce'
a matrix to a row vector, or a row vector to a scalar.  +/A  produces a row
vector containing the row sums of A if A is a matrix.  If A is a row vector, it
produces the sum of the elements of A.  Similiarly, -/ reduces by an alternating
summation, */ by multiplication, and // by division.

The operator +\ produces a cumulative row summation.  The result is the same
size as the argument.  For X = +\A,  X(i,j) = A(i,1) + ... + A(i,j).
+/A would be the last column of X transposed.

The operator *\ produces a cumulative row product.  The result is the same
size as the argument.  For X = *\A,  X(i,j) = A(i,1) * A(i,2) * ...  A(i,j).
*/A would be the last column of X transposed.