In this paper, we will consider the generating operator of the free group
factor L(F_2). Then we can construct the group von Neumann algebra L(K), where
K is the commutator group of F_2 and the conditional expectation E. Then
(L(F_2), E) is the W*-probability space with amalgamation over L(K). In this
paper, we will compute the trivial operator-valued moment series of the
generating operator of L(F_2) over L(K). This computation is the good example
for studying the operator-valued distribution, since the operator-valued moment
series of operator-valued random variables contain algebraic and combinatorial
free probability information about the opeartor-valued distributions.