[numerics.defns]

Define GENERALIZED_NONCOMMUTATIVE_SUM(op, a1, …, aN) as follows:

(1.1)
a1 when N is 1, otherwise
(1.2)
op(GENERALIZED_NONCOMMUTATIVE_SUM(op, a1, …, aK),

op(GENERALIZED_NONCOMMUTATIVE_SUM(op, aM, …, aN)) for any K where $1 < K + 1 = M \leq N .$

Define GENERALIZED_SUM(op, a1, …, aN) as GENERALIZED_NONCOMMUTATIVE_SUM(op, b1, …, bN),
where b1, …, bN may be any permutation of a1, …, aN.