  We prove a determinantal identity concerning Schur functions for 2-staircase
diagrams lambda=(ln+l',ln,l(n-1)+l',l(n-1),...,l+l',l,l',0). When l=1 and l'=0
these functions are related to the partition function of the 6-vertex model at
the combinatorial point and hence to enumerations of Alternating Sign Matrices.
A consequence of our result is an identity concerning the doubly-refined
enumerations of Alternating Sign Matrices.
