theorem Th11:
  p is_subformula_of p '&' q & q is_subformula_of p '&' q
proof
A1: q is_proper_subformula_of p '&' q by QC_LANG2:69;
  p is_proper_subformula_of p '&' q by QC_LANG2:69;
  hence thesis by A1,QC_LANG2:def 21;
end;
