  In 1993, Samko and Ross introduced the study of fractional integration and
differentiation when the order is not a constant but a function. This
suggestion gave rise to a number of further ideas and results. In particular,
this implies a possibility of integration with respect to derivative's order.
Here an identity is presented, in which an expression involving
Riemann--Liouville fractional derivative is integrated with respect to the
derivative's order.
