%% usm.gl
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%  2-Counter machine
%%    Example: R1 + R2 -> R1    
%%    1: jpz(R2,5,2)
%%    2: dec(R2)
%%    3: inc(R1)
%%    4: goto(1)
%%    5: halt
%%
true =>
   % program
   instruction(1,jpz,2,5,2),
   instruction(2,dec,2,3,x),
   instruction(3,inc,1,4,x),
   instruction(4,goto,1,x,x),
   instruction(5,halt,x,x,x),
   % data
   inc(0,1), inc(1,2), inc(2,3), inc(3,4),  
   state(1,3,4).  // START compute 3 + 4

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%  UNIVERSAL INTERPRETATION OF REGISTER 
%%  INSTRUCTIONS
%-- jpz
state(PC,0,R2), 
   instruction(PC,jpz,1,PA,PB)
       =>  state(PA,0,R2).
state(PC,R1,R2), 
   inc(_X,R1),  % R1 is NOT zero 
   instruction(PC,jpz,1,PA,PB) 
      => state(PB,R1,R2).
state(PC,R1,0), 
   instruction(PC,jpz,2,PA,PB) 
      => state(PA,R1,0).
state(PC,R1,R2), 
   inc(_,R2), % R2 is NOT zero
   instruction(PC,jpz,2,PA,PB) 
      => state(PB,R1,R2).
%-- dec
state(PC,R1,R2), 
   instruction(PC,dec,1,PB,x), 
   inc(D,R1) 
      => state(PB,D,R2).
state(PC,R1,R2), 
   instruction(PC,dec,2,PB,x), 
   inc(D,R2) 
      => state(PB,R1,D).
%-- inc
state(PC,R1,R2), 
   instruction(PC,inc,1,PB,x), 
   inc(R1,I) 
      => state(PB,I,R2).
state(PC,R1,R2), 
   instruction(PC,inc,2,PB,x), 
   inc(R2,I) 
      => state(PB,R1,I).
%-- goto
state(PC,R1,R2), 
   instruction(PC,goto,PB,x,x) 
      => state(PB,R1,R2).
%-- halt
state(PC,R1,R2), 
   instruction(PC,halt,x,x,x) 
      => goal.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%  Natural number generation via inc
%%  
inc(X,Y) => inc(Y,SomeZ).