theorem
  M,v |= 'not' p => (p => q) & M |= 'not' p => (p => q)
proof
  now
    let v;
    now
      assume M,v |= 'not' p;
      then not M,v |= p by ZF_MODEL:14;
      hence M,v |= p => q by ZF_MODEL:18;
    end;
    hence M,v |= 'not' p => (p => q) by ZF_MODEL:18;
  end;
  hence thesis;
end;
