let n be Nat; :: thesis: ( not n <= 4 or n = 0 or n = 1 or n = 2 or n = 3 or n = 4 )
assume n <= 4 ; :: thesis: ( n = 0 or n = 1 or n = 2 or n = 3 or n = 4 )
then n <= 3 + 1 ;
hence ( n = 0 or n = 1 or n = 2 or n = 3 or n = 4 ) by Th8, Th28; :: thesis: verum