let n be even Nat; :: thesis: ( n <= 1 implies n = 0 )
assume A1: n <= 1 ; :: thesis: n = 0
n <> (2 * 0) + 1 ;
then n < 0 + 1 by A1, XXREAL_0:1;
hence n = 0 by NAT_1:13; :: thesis: verum