let n be Nat; :: thesis: ( not n < 4 or n = 0 or n = 1 or n = 2 or n = 3 )
assume n < 4 ; :: thesis: ( n = 0 or n = 1 or n = 2 or n = 3 )
then n < 3 + 1 ;
then n <= 3 by NAT_1:13;
then not not n = 0 & ... & not n = 3 ;
hence ( n = 0 or n = 1 or n = 2 or n = 3 ) ; :: thesis: verum