let n be Nat; :: thesis: ( not n <= 3 or n = 0 or n = 1 or n = 2 or n = 3 )
assume n <= 3 ; :: thesis: ( n = 0 or n = 1 or n = 2 or n = 3 )
then n <= 2 + 1 ;
hence ( n = 0 or n = 1 or n = 2 or n = 3 ) by Th8, Th27; :: thesis: verum