reserve i,n,m,k,x,y for Nat,
  i1 for Integer;

theorem Th4:
  2 <= k & m is_represented_by 1,k implies DigA_SDSub(SD2SDSub(
  DecSD(m,1,k)),1+1) = SDSub_Add_Carry(m, k)
proof
  assume that
A1: 2 <= k and
A2: m is_represented_by 1,k;
  thus DigA_SDSub(SD2SDSub(DecSD(m,1,k)),1+1) = SDSub_Add_Carry( DigA(DecSD(m,
  1,k),1), k) by A1,Th3
    .= SDSub_Add_Carry(m, k) by A2,RADIX_1:21;
end;
