reserve p,q,r,s,A,B for Element of PL-WFF,
  F,G,H for Subset of PL-WFF,
  k,n for Element of NAT,
  f,f1,f2 for FinSequence of PL-WFF;
reserve M for PLModel;
reserve x,y for set;

theorem
  A is tautology iff {}PL-WFF |- A by tautsat,sth,ct;
