reserve x,y,c for set;

theorem
  InnerVertices BitSubtracterWithBorrowStr(x,y,c) is Relation
proof
  InnerVertices 2GatesCircStr(x,y,c, 'xor') is Relation & InnerVertices
  BorrowStr(x,y,c) is Relation by Th1,FACIRC_1:58;
  hence thesis by FACIRC_1:3;
end;
