theorem Th51:
  H is_immediate_constituent_of F implies len @H < len @F
proof
A1: F = VERUM(A) or F is atomic or F is negative or F is conjunctive or F is
  universal by QC_LANG1:9;
  assume H is_immediate_constituent_of F;
  then F is negative & H = the_argument_of F or F is conjunctive & H =
  the_left_argument_of F or F is conjunctive & H = the_right_argument_of F or F
  is universal & H = the_scope_of F by A1,Th41,Th47,Th48,Th49,Th50;
  hence thesis by QC_LANG1:14,15,16;
end;
