A recursive definition for the language AtLeast$3$EndBB over the alphabet $=\{ab\}$ must be compiled where AtLeast$3$EndBB has as elements all words that have at least three characters and end with a $bb-$substring. The generators are abb and $bbb$ and the CONCAT function is applied in defining a recursive definition for the language. We begin our definition with:

"AtLeast$3$EndBB is the smallest subset of $\{ab{\}}^{**}$ such that abb,bbbin AtLeast$3$EndBB and if win AtLeast$3$EndBB, then $..."$

Certain CONCAT operations which can generate all possible words belonging to AtLeast$3$EndBB should now be provided.