sentences:  If entry were an equilibrium, the earlier steps would form a pre-equilibrium and the specificity benefits of entry into the pathway (less likely for the substrate analogue) would be lost; if the exit step were an equilibrium, then the substrate analogue would be able to re-enter the pathway through the exit step, bypassing the specificity of earlier steps altogether.Although one test will only be able to discriminate between mismatched and matched sequences a fraction formula_2 of the time, two tests will both fail only formula_3 of the time, and N tests will fail formula_4 of the time.In terms of free energy, the discrimination power of N successive tests for two states with a free energy formula_5 is the same as one test between two states with a free energy formula_6.