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+[ ROT13 in Brainfuck
+from https://en.wikipedia.org/wiki/Brainfuck#ROT13
+]
+
+-,+[                         Read first character and start outer character reading loop
+-[                       Skip forward if character is 0
+>>++++[>++++++++<-]  Set up divisor (32) for division loop
+(MEMORY LAYOUT: dividend copy remainder divisor quotient zero zero)
+<+<-[                Set up dividend (x minus 1) and enter division loop
+>+>+>-[>>>]      Increase copy and remainder / reduce divisor / Normal case: skip forward
+<[[>+<-]>>+>]    Special case: move remainder back to divisor and increase quotient
+<<<<<-           Decrement dividend
+]                    End division loop
+]>>>[-]+                 End skip loop; zero former divisor and reuse space for a flag
+>--[-[<->+++[-]]]<[         Zero that flag unless quotient was 2 or 3; zero quotient; check flag
+++++++++++++<[       If flag then set up divisor (13) for second division loop
+(MEMORY LAYOUT: zero copy dividend divisor remainder quotient zero zero)
+>-[>+>>]         Reduce divisor; Normal case: increase remainder
+>[+[<+>-]>+>>]   Special case: increase remainder / move it back to divisor / increase quotient
+<<<<<-           Decrease dividend
+]                    End division loop
+>>[<+>-]             Add remainder back to divisor to get a useful 13
+>[                   Skip forward if quotient was 0
+-[               Decrement quotient and skip forward if quotient was 1
+-<<[-]>>     Zero quotient and divisor if quotient was 2
+]<<[<<->>-]>>    Zero divisor and subtract 13 from copy if quotient was 1
+]<<[<<+>>-]          Zero divisor and add 13 to copy if quotient was 0
+]                        End outer skip loop (jump to here if ((character minus 1)/32) was not 2 or 3)
+<[-]                     Clear remainder from first division if second division was skipped
+<.[-]                    Output ROT13ed character from copy and clear it
+<-,+                     Read next character
+]                            End character reading loop