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 Th60:
  Free ('not' p) = Free p
proof
  'not' p is negative & the_argument_of 'not' p = p by Th3;
  hence thesis by ZF_MODEL:1;
end;
