Return vertex coordinates for CHUNK of 3D mesh M3. The CHUNK may be a list of cell indices, in which case xyz3 returns a (CHUNK.shape))x3x2x2x2 list of vertex coordinates. CHUNK may also be a mesh-specific data structure used in the slice3 routine (see "slice3: Plane and Isosurface Slices of a 3-D mesh" on page 65), in which case xyz3 may return a 3x(ni)x(nj)x(nk) array of vertex coordinates. For meshes which are logically rectangular or consist of several rectangular patches, this is up to 8 times less data, with a concomitant performance advantage. Use xyz3 when writing slicing functions or coloring functions for slice3.