  Differential expansion (DE) for a Wilson loop average in representation $R$
is built to respect degenerations of representations for small groups. At the
same time it behaves nicely under some changes of the loop, e.g. of some knots
in the case of $3d$ Chern-Simons theory. Especially simple is the relation
between the DE for the trefoil $3_1$ and for the figure eight knot $4_1$. Since
arbitrary colored HOMFLY for the trefoil are known from the Rosso-Jones
formula, it is therefore enough to find their DE in order to make a conjecture
for the figure eight. We fulfil this program for all rectangular representation
$R=[r^s]$, i.e. make a plausible conjecture for the rectangularly colored
HOMFLY of the figure eight knot, which generalizes the old result for totally
symmetric and antisymmetric representations.
