  Value of generalized hypergeometric function at a special point is
calculated. More precisely, value of certain multiple integral over vanishing
cycle (all arguments collapse to unity) is calculated. The answer is expressed
in terms of $\Gamma$-functions. The constant is relevant to the part of $\rho$
in the Gindikin-Karpelevich formula for c-function of Harish-Chandra.
Calculation is an adaptation of classical calculations of Gelfand and Naimark
(1950) to the Heckman-Opdam hypergeometric functions in the case of root system
of type $A_{n-1}$.
