Besides CASP tools' strengths, their key bottleneck is computational complexity. During retrosynthesis planning runtime, potentially exponential in size search space of solution candidates (partial synthetic routes) must be traversed. It makes CASP tools nonapplicable when numerous molecules need to be immediately checked for synthesizability. One example is a virtual screening (VS) method known in computerassisted drug design (CADD). During VS, even billions of compound candidates are evaluated for desired properties; thus, searching for a synthetic route for each of these candidates is computationally intractable.