let N be Pnet; :: thesis: for x being Element of Elements N st Elements N <> {} holds
enter (N,x) c= Elements N
let x be Element of Elements N; :: thesis: ( Elements N <> {} implies enter (N,x) c= Elements N )
assume A1: Elements N <> {} ; :: thesis: enter (N,x) c= Elements N
A2: ( enter (N,x) = {x} implies enter (N,x) c= Elements N )
proof
x in Elements N by A1;
then A3: for y being object st y in {x} holds
y in Elements N by TARSKI:def 1;
assume enter (N,x) = {x} ; :: thesis: enter (N,x) c= Elements N
hence enter (N,x) c= Elements N by A3, TARSKI:def 3; :: thesis: verum
end;
( enter (N,x) = {x} or enter (N,x) = Pre (N,x) ) by A1, Th15;
hence enter (N,x) c= Elements N by A2, Th9; :: thesis: verum