The leaf node now contains just 7.

Repair strategy: try to borrow values from a neighbouring node.

We join together the current node and a neighbour to form a combined node.

Don't forget to include the value between these 2 adjacent subtrees.

We choose to join [7] with [17,22,44,45]:

[6,7,10,17,22,44,45]

• parent contributes 1 value.
• node that underflowed: (M-1)/2 - 1 values.
• neighbour: between (M-1)/2 and (M-1) values.

We distinguish 2 cases, depending on whether the neighbour contributes exactly (M-1)/2 values or more.