ringmap

f mapTable ringmap r
f is mapped to r where the map is defined by the mapTable, which is generated
by makeRingMap as follows:
 rule  ring1 ring2 makeRingMap maptable   
Example:
[(x,y) ring_of_differential_operators ( ) elimination_order 0] define_ring
/R1 set
[(t,y,z) ring_of_differential_operators ( ) elimination_order 0] define_ring
/R2 set
[[(x) (Dx)] [((t-1) Dt) (z)]] /r0 set
r0 R1 R2 makeRingMap /maptable set
(Dx-1) R1 ,, /ff set
 ff maptable ringmap ::