: Macro Operators
!!! OBSOLETE !!!
This command has not been maintained since 1996. So, there may be
troubles if it is used with new features of kan/sm1.
This command is used to define a ring of difference operators
with the macros define_ring, weight_vector and elimination_order.
The user cannot use the variable names h, e, H and E.
cf. show_ring, system_variable --- CurrentRing
Example: [(x,y,z) ring_of_difference_operators (Ex,Ey) elimination_order 0] define_ring
Ex,Ey, and Ez are corresponding difference operators to the
space variables x, y and z ( Ex x = (x+1) Ex).