Description: The associative law for binary operations, see definitions of laws A1.
and M1. in section 1.1 of Hall p. 1, or definition 5 in BourbakiAlg1
p. 4: the value of a binary operation applied the value of the binary
operation applied to two operands and a third operand equals the value
of the binary operation applied to the first operand and the value of
the binary operation applied to the second and third operand. By this
definition, the associative law is expressed as binary relation: a
binary operation is related to a set by assLaw if the associative
law holds for this binary operation regarding this set. Note that the
binary operation needs neither to be closed nor to be a function.
(Contributed by FL, 1-Nov-2009)(Revised by AV, 13-Jan-2020)