  Small angle neutron diffraction experiments are analyzed using recently
developed and properly generalized one-field effective free energy method. In
the case of experiment of Keimer et al on YBCO, we show that the fourfold
symmetry of the underlying crystal is explicitly broken, but the reflection
with respect to the [110] and [1\={1}0] axes remains a symmetry. The vortex
lattice also becomes generally oblique instead of rectangular body centered.
Unexpectedly rich phase diagram is described.
