reserve a,b,c for boolean object;

theorem Th1:
  (a=>(b '&' c))=>(a=>b)=1
 proof
  A1: b=0 or b=1 by XBOOLEAN:def 3;
  a=0 or a=1 by XBOOLEAN:def 3;
  hence (a=>(b '&' c))=>(a=>b)=1 by A1;
 end;
