(This symbol permutes a product of noncommuting operators of the form B 1 ( t 1 ) ⋅ ⋅ B 2 ( t 2 ) ⋅ ⋅ ⋯ ⋯ ⋅ ⋅ B n ( t n ) into the uniquely determined re-ordered expression B i 1 ( t i 1 ) ⋅ ⋅ B i 2 ( t i 2 ) ⋅ ⋅ ⋯ ⋯ ⋅ ⋅ B i n ( t i n ) with t i 1 ≥ ≥ t i 2 ≥ ≥ ⋯ ⋯ ≥ ≥ t i n .