reserve k,m,n for Nat, kk,mm,nn for Element of NAT, X,Y,x,y,z for set;
reserve S,S1,S2 for Language, s,s1,s2 for Element of S;

theorem Th12:
x is string of S iff x is non empty Element of ((AllSymbolsOf S)*)
proof
set SS=AllSymbolsOf S;
x is string of S iff (x in SS* & not x in {{}}) by XBOOLE_0:def 5;
hence thesis;
end;
