I am reading the book "Young tableaux with application" by Fulton W. There is claim says that
"The product operation makes the set of tableaux into an associative monoid"
Where the product operation between two tableaux $T$ and $U$ is defined as follows:
$T*U$ is determined by starting with $T$ and row-insert the left-most entry in the bottom row of $U$ into T. Row-insert into the result the next entry of the bottom row $U$, and continue until all entries in the bottom row of $U$ have been inserted. Then insert the next bottom row of $U$ in the same way and we can get $T*U$ once we insert all of the boxes in $U$
And my question is that how can I prove that this operation is associative?