Description: Associative law for inner product. Postulate (S3) of Beran p. 95.
Warning: Mathematics textbooks usually use our version of the axiom.
Physics textbooks, on the other hand, usually replace the left-hand side
with ( B .ih ( A .h C ) ) (e.g., Equation 1.21b of Hughes p. 44;
Definition (iii) of ReedSimon p. 36). See the comments in df-bra for
why the physics definition is swapped. (Contributed by NM, 29-May-1999)(New usage is discouraged.)