reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;

theorem Th58:
  Free (x '=' y) = {x,y}
proof
A1: Var2 (x '=' y) = y by Th1;
  x '=' y is being_equality & Var1 (x '=' y) = x by Th1;
  hence thesis by A1,ZF_MODEL:1;
end;
