simple question on mesh_snap_boundary

Hi all,

I explored the usage of "mesh_snap_boundary" with the following simple code:

### Beginning of code
mesh=UnitSquare(2,2)
print mesh.coordinates()

class LeftRegion(SubDomain):
 def inside(self, x, on_boundary):
  return True if x[0]<=0.5 else False
 def snap(self, x):
  if x[0]<=0.5+DOLFIN_EPS: x[0]=0.5*(x[0]-0); x[1]=x[1]

left = LeftRegion()
mesh.snap_boundary(left)
print mesh.coordinates()
### End of code

I simply want to push the vertices on the left side of the unit square towards y-axis by a factor of 0.5, so [0.5, y] should become [0.25, y]. However, the output of the code above is, after "snapping",
"""

[[ 0. 0. ]
 [ 0.25 0. ]
 [ 1. 0. ]
 [ 0. 0.5 ]
 [ 0.375 0.5 ]
 [ 1. 0.5 ]
 [ 0. 1. ]
 [ 0.25 1. ]
 [ 1. 1. ]]
"""

Could anyone tell me why the center vertex became [0.375, 0.5], rather than [0.25, 0.5]?

Thanks a lot!