[Hughes and Cresswell, 1996]
The system S4.2.1 is the result of S4.2 + N1 [ ((p=>Lp)=>p)>(MLp>Lp) ] [Hughes and Cresswell, 1996, p263]
OR S4.2.1 is S4.2 + N1S [ ((p=>Lp)=>p)=>(MLp>p) ] [Zeman, 1973]
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