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 Th4:
  the_left_argument_of (p '&' q) = p & the_right_argument_of (p '&' q) = q
proof
  p '&' q is conjunctive;
  then
  p '&' q = (the_left_argument_of (p '&' q)) '&' (the_right_argument_of (p
  '&' q)) by ZF_LANG:40;
  hence thesis by ZF_LANG:30;
end;
