  We report large-scale quantum Monte Carlo calculations at the T=0
antiferromagnetic to valence-bond-solid (VBS) transition of a two-dimensional
S=1/2 XY model with four-spin interactions. Finite-size scaling suggests a
discontinuous spin stiffness, but a continuous VBS order-parameter. We propose
that this is a continuous VBS transition without critical spin fluctuations. We
also argue that the system is close to a point, in an extended parameter space,
where both the magnetic and VBS fluctuations are critical--possibly a
deconfined quantum-critical point.
