let n be Nat; :: thesis: ( not n < 3 or n = 0 or n = 1 or n = 2 )
assume n < 3 ; :: thesis: ( n = 0 or n = 1 or n = 2 )
then n < 2 + 1 ;
then n <= 2 by NAT_1:13;
then not not n = 0 & ... & not n = 2 ;
hence ( n = 0 or n = 1 or n = 2 ) ; :: thesis: verum