theorem Th26:
  for X being set st X c= bound_QC-variables(Al) holds not x in X
  implies v.(x|a)|X = v|X
proof
  let X be set such that
A1: X c= bound_QC-variables(Al) and
A2: not x in X;
  set f2 = v|X;
  set f1 = v.(x|a)|X;
A3: dom f1 = dom f2 by A1,SUBLEMMA:63;
  now
    let b be object such that
A4: b in dom f1;
    x <> b by A2,A4;
    then
A5: v.(x|a).b = v.b by SUBLEMMA:48;
    v.(x|a).b = f1.b by A4,FUNCT_1:47;
    hence f1.b = f2.b by A3,A4,A5,FUNCT_1:47;
  end;
  hence thesis by A3,FUNCT_1:2;
end;
