reserve i for Nat,
  j for Element of NAT,
  X,Y,x,y,z for set;

theorem Th27:
  for x being variable holds varcl {x} = (vars x) \/ {x}
proof
  let x be variable;
  x in Vars;
  then consider A being Subset of Vars, j such that
A1: x = [varcl A, j] and A is finite by Th18;
  varcl {x} = (varcl A) \/ {x} by A1,Th26;
  hence thesis by A1;
end;
