Embedding of the atomic theory of subsets of free semigroups to the atomic theory of subsets of free monoids

In this paper, we consider atomic formulas constructed from the binary predicate symbol $\subseteq$ and binary function symbols $\backslash$, $/$, $\cup$, and $\cap$. For $X$ and $Y$ from the powerset of a free semigroup, $X/Y$ denotes the set consisting of elements whose product with any element of $Y$ ( multiplying on the right) belongs to $X$. Similarly, one defines $Y \backslash X$ (multiplying on the left). We prove that every atomic formula that is true in every free semigroup powerset interpretation is also true in every free monoid powerset interpretation.