ch

ch is the abbreviation of characteristic.
 a ch b
 array a;  number b
b is the generator of the characteristic variety of a.
For the algorithm, see, Japan J. of Industrial and Applied Math., 1994, 485--497.
Example 1 : 
 [ [( (x Dx)^2 + ( y Dy)^2) ( x Dx y Dy -1)] (x,y)] ch :: 
Example 2 : 
[ [( (x^3-y^2) Dx + 3 x^2) ( (x^3-y^2) Dy - 2 y)] (x,y)] ch ::