augment.dot

strict digraph {
l1 [label="[1, 2]" shape=box];
l2 [label="[2]" shape=box];
p -> l1;
op -> l1;
q -> l2;
label = "after p += q";
ordering = out;
}

before.dot

strict digraph {
p;
l1 [label="[1]" shape=box];
l2 [label="[2]" shape=box];
p -> l1;
op -> l1;
q -> l2;
label = "before"
ordering = out;
}

fresh.dot

strict digraph {
ls [label="[1, 2]" shape=box];
l1 [label="[1]" shape=box];
l2 [label="[2]" shape=box];
p -> ls;
op -> l1;
q -> l2;
label = "after p = p + q";
ordering = out;
}