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immutable_rules/102_Mutability.md

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+Initially rules in the 100's are immutable and rules in the 200's are mutable.
+Rules subsequently enacted or transmuted (that is, changed from immutable to
+mutable or *vice versa*) may be immutable or mutable regardless of their
+numbers, and rules in the Initial Set may be transmuted regardless of their
+numbers.
+
+If the game is being played with git, mutable rules must be stored in the
+`mutable_rules/` directory and immutable rules must be stored in the
+`immutable_rules/` directory. The directory in which a rule resides is the
+sole factor in determining its mutability.