S7 (Halldén) is 20sa (Pledger)[Pledger, 1972, 277]
[Hughes and Cresswell, 1996, p207] say that S3 is the intersection of S4 and S7 (In the sense that a theorem is a theorem of S3 if, and only if, it is a theorem of both S4 and S7.)
The system S7 is:
S3 + MMp [S] [Halldén, 1950, p230] [Hughes and Cresswell, 1996, p364]
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