Leftist grammar

In formal language theory, a leftist grammar is a formal grammar on which certain restrictions are made on the left and right sides of the grammar's productions. Only two types of productions are allowed, namely those of the form $$a \to ba$$ (insertion rules) and $$cd \to d$$ (deletion rules). Here, $$a,b,c$$ and $$d$$ are terminal symbols. This type of grammar was motivated by accessibility problems in the field computer security.

Computational properties
The membership problem for leftist grammars is decidable.