theorem Th7:
  for N being Neighbourhood of z0 holds z0 in N
proof
  let N be Neighbourhood of z0;
  consider g be Real such that
A1: 0 < g and
A2: {z where z is Complex : |.z-z0.| < g} c= N by Def5;
  |.z0-z0.| = 0 by COMPLEX1:44;
  then z0 in {z where z is Complex : |.z-z0.| < g} by A1;
  hence thesis by A2;
end;
