theorem Th13:
  for l being CQC-variable_list of k,A, i st 1<=i & i<=len l holds l
  .i in bound_QC-variables(A)
proof
  let l be CQC-variable_list of k,A, i;
  assume that
A1: 1<=i and
A2: i<=len l;
  i in dom l by A1,A2,FINSEQ_3:25;
  then
A3: l.i in rng l by FUNCT_1:def 3;
  rng l c= bound_QC-variables(A) by RELAT_1:def 19;
  hence thesis by A3;
end;
