reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;
reserve F,F1,G,G1,H,H1 for ZF-formula;
reserve sq,sq9 for FinSequence;

theorem Th57:
  H is conjunctive implies (F is_immediate_constituent_of H iff F
  = the_left_argument_of H or F = the_right_argument_of H)
proof
  assume H is conjunctive;
  then H = (the_left_argument_of H) '&' the_right_argument_of H by Th40;
  hence thesis by Th53;
end;
