What is the recommended way to redefine an expression?

[tueda]

Is there any simple way to redefine an expression without (all)warnings? The following code gives a warning for replacing the expression F, though I have explicitly declared F will be dropped unless any redefinition is given:

On allwarnings;
L F = 2;
.sort
Drop F;
L F = 3 * F;
P;
.end

FORM 4.2.0 (Oct 26 2017, v4.2.0-22-g954024f) 64-bits  Run: Fri Nov 24 13:19:05 2017
    On allwarnings;
    L F = 2;
    .sort

Time =       0.00 sec    Generated terms =          1
               F         Terms in output =          1
                         Bytes used      =         20
    Drop F;
    L F = 3 * F;
test.frm Line 5 --> Warning: Expression is replaced by new definition
    P;
    .end

Time =       0.00 sec    Generated terms =          1
               F         Terms in output =          1
                         Bytes used      =         20

   F =
      6;

In principle, one can copy the expression to a temporary expression like

On allwarnings;
L F = 2;
.sort
Drop F;
L tmp = F;
.sort
Drop tmp;
L F = 3 * tmp;
P;
.end

but it looks nasty and has some overheads.