theorem Th18:
  Al is countable implies
  QC-WFF(Al) is countable
proof
  assume A1: Al is countable;
  QC-WFF(Al) is Al-closed by QC_LANG1:def 11;
  then
A2: QC-WFF(Al) is Subset of [: NAT, QC-symbols(Al):]* by QC_LANG1:def 10;
  [:NAT,QC-symbols(Al):] is non empty set &
  [:NAT,QC-symbols(Al):] is countable by A1,QC_LANG1:5;
  then [:NAT,QC-symbols(Al):]* is countable by CARD_4:13;
  hence thesis by A2;
end;
