Physically the only relevant (measurable) quantity is

F.

Since

F=2d

A, adding d

to A gives:

A' =

A + d

F' = 2d

A' = 2d

A + 2d(d

)

... but d(d(

anything)) is zero (by the definition of the 'd' operator - see p. 232) so:

F' = 2d

A =

F...i.e. adding d

leaves

F unchanged!