theorem Th41:
  p is closed & p |- q implies p => q is valid
proof
  assume that
A1: p is closed and
A2: p |- q;
  {}(CQC-WFF(A)) \/ {p} |- q by A2;
  then {}(CQC-WFF(A)) |- p => q by A1,CQC_THE2:92;
  hence thesis by CQC_THE1:def 9;
end;
