  We study Schubert polynomials using geometry of infinite-dimensional flag
varieties and degeneracy loci. Applications include Graham-positivity of
coefficients appearing in equivariant coproduct formulas and expansions of
back-stable and enriched Schubert polynomials. We also construct an embedding
of the type C flag variety, and study the corresponding pullback map on
(equivariant) cohomology rings.
