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[buran]

a is 81-char string, i.e. 81 digits as string. a[:i]+m+a[i+1:] will replace the char (digit on index i) with m

that is not 100% precise
r(a[:i]+m+a[i+1:]) is calling function r with a string that is derived from a, but the char (digit) on index i is replaced with m