There are no minimal essentially undecidable theories

Summary: This passage describes the formulation of two theories, WD and D, in a specific language where constant, function, and relation symbols are used. These theories are mutually interpreted with arithmetic theories R and Q respectively. The intended model for WD and D is generated by certain elements in a free semigroup, extended with string concatenation and the prefix relation. Unlike Q, WD and D are purely universally axiomatized.