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 Th59:
  Free (x 'in' y) = {x,y}
proof
A1: Var2 (x 'in' y) = y by Th2;
  x 'in' y is being_membership & Var1 (x 'in' y) = x by Th2;
  hence thesis by A1,ZF_MODEL:1;
end;
