reserve a,b,c for boolean object;

theorem Th2:
  (a=>(b=>c))=>((a '&' b)=>c)=1
 proof
  A1: a=0 or a=1 by XBOOLEAN:def 3;
  A2: b=0 or b=1 by XBOOLEAN:def 3;
  c =0 or c =1 by XBOOLEAN:def 3;
  hence thesis by A1,A2;
 end;
