Ordinal semigroups

In a previous paper we introduced a version of associativity for a partial infinitary operation. We prove here that if $\gamma$ is an infinite ordinal and some associative infinitary operation is defined for all sequences indexed by ordinals $\leq \gamma$, then such an operation can be uniquely expanded to apply to every sequence indexed by any ordinal of cardinality $|\gamma |$. In particular, if some associative operation is defined for all finite sequences as well as for all $\omega$-indexed sequences, then the operation can be uniquely expanded to apply to every sequence indexed by a countable ordinal.