Solution
= Solution
Let
$$
G=(\{a,b\},\{S\},\{S\to a\},S),
\qquad
G'=(\{a,b\},\{T\},\{T\to b\},T).
$$
Both are <regular grammar>s and their variable sets are disjoint. Their ordinary concatenation grammar adds a new rule
$$
S_0\to ST.
$$
This rule has two variables on its right-hand side, so it is not right-linear. Thus this is an explicit instance in which the <concatenation grammar need not be regular>.