The addition operation in the domain of multiindices $Z$ is written $+$.
Let $k = \sequence {k_j}_{j \mathop \in J}$ and $l = \sequence {l_j}_{j \mathop \in J}$ be multiindices.
Then $k + l$ is defined as:
$\paren {\forall j \in J} \paren {\paren {k + l}_j = k_j + l_j}$