ambnambn.lmn: this file is (C) Copyright 2006 Peri Hankey (mpah@users.sourceforge.net). This text is published under the terms of the Gnu General Program License, and comes with absolutely no warranty.

{a^m*b^n*a^m*b^n, where a and b are terminal & ((m = 0 & n = 0) | n > 0 & m > 0) & (no variables permitted)}

Test harness - S may be empty, so require terminator

 esc out           <- eof - ;              // H1
 .[ \n]            <- eof - ;              // H2
 - _E              <- eof - esc "no\n" ;   // H3 
 - _S ';'          <- eof - esc "yes\n";   // H4 

 - {repeat .[^\n]} <- _E ;                 // E1

The null case, and a game of two halves

 -                 <- _S ;                 // S01 
 - _P _Q           <- _S ;                 // S02 

Recognise 'a'^m*'b'^n (n > 0, m > 0), transform to _a^m*_b*n

 'a' _A _B         <- _P _a;               // S03 

 -                 <- _A ;                 // S04
 'a' _A _B         <- _A _a;               // S05

 -                 <- _B ;                 // S06
 'b' _B            <- _B _b;               // S07

Recognise one 'b' for each _b:
NB will only see actual input at end of recursion for _Y after matching all _b and all _a.
Effective input eg _a _a _b 'aab' is to be transformed and analysed as _b _a _a 'aab'.
So we need to go top down to look for _b

 - _b  _Y 'b'      <- _Q ;                 // S08

 - _b  _Y 'b'      <- _Y ;                 // S10
 _a  _Z 'a'        <- _Y ;                 // S11

 -                 <- _Z ;                 // S12
 _a  _Z 'a'        <- _Z ;                 // S13

Treat _a _b as _b _a when looking for _b

 _a  _b            <- _b _a ;              // S14