Brzozowski and Leiss[REF] studied left language equations where every concatenation is with a singleton constant language on the left, e.g.
Baader and Narendran[REF] studied equations F(X_1, \ldots, X_k)=G(X_1, \ldots, X_k) using left-concatenation and union and proved that their satisfiability problem is EXPTIME-complete.