Common block cisnas
contains the integers ileft, iright, idown
and iup that indicate the sequence numbers (1..4) of the different faces
of a subdomain in 2-D. The advantage of storing these constants in a common
block is that it results in
more readable code.
Also code written using these constants does not
need to be changed when the numbering of the faces changes. Only
the common block cisnas needs to be changed in such cases.