  We present a new method of proof, which we call reverse induction. This
method allows to establish certain properties of a product
$\prod_{{i}=0}^{\infty}{X}_{i}$ by making a kind of "reverse induction step"
from $\prod_{{i}={n}+1}^{\infty}{X}_{i}$ to $\prod_{{i}={n}}^{\infty}{X}_{i}$
for an arbitrary natural ${n}$. Using this method we answer a question posed by
E. K. van Douwen and W. F. Pfeffer in 1979 by proving that the countable power
of the Sorgenfrey line is a D-space.
