/*REXX program displays (all) the  sums  of a  full  4─bit adder  (with carry).         */
call hdr1;  call hdr2                            /*note the order of headers & trailers.*/
                                                 /* [↓]  traipse thru all possibilities.*/
   do    j=0  for 16
                            do m=0  for 4;   a.m=bit(j, m);   end  /*m*/
      do k=0  for 16
                            do m=0  for 4;   b.m=bit(k, m);   end  /*m*/
      sc=4bitAdder(a., b.)
      z=a.3 a.2 a.1 a.0   '_+_'   b.3 b.2 b.1 b.0    "_=_"    sc    ','    s.3 s.2 s.1 s.0
      say translate(space(z, 0), , '_')          /*remove all the underbars (_) from Z. */
      end   /*k*/
   end      /*j*/

call hdr2;  call hdr1                            /*display two trailers (note the order)*/
exit                                             /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
bit:       procedure;  parse arg x,y;    return  substr( reverse( x2b( d2x(x) ) ), y+1, 1)
halfAdder: procedure expose c;   parse arg x,y;      c=x & y;     return x && y
hdr1:      say 'aaaa + bbbb = c, sum     [c=carry]';              return
hdr2:      say '════   ════   ══════'              ;              return
/*──────────────────────────────────────────────────────────────────────────────────────*/
fullAdder: procedure expose c;   parse arg x,y,fc
           _1=halfAdder(fc, x);        c1=c
           _2=halfAdder(_1, y);        c=c | c1;                  return _2
/*──────────────────────────────────────────────────────────────────────────────────────*/
4bitAdder: procedure expose s. a. b.;  carry.=0
                                do j=0  for 4;                n=j-1
                                s.j=fullAdder(a.j, b.j, carry.n);      carry.j=c
                                end   /*j*/
           return c