  The ring of symmetric functions can be implemented in the homology of
\union_{a,b} Gr(a,a+b), the multiplicative structure being defined from the
"direct sum" map. There is a natural circle action (simultaneously on all
Grassmannians) under which each direct sum map is equivariant. Upon replacing
usual homology by equivariant K-homology, we obtain a 2-parameter deformation
of the ring of symmetric functions.
  This ring has a module basis given by Schubert classes. Geometric
considerations show that multiplication of Schubert classes has positive
coefficients, in an appropriate sense. In this paper we give manifestly
positive formulae for these coefficients: they count numbers of "DS pipe
dreams'' with prescribed edge labelings.
