If you don't mind c being a variable, you could do it this way (it
should have been documented under "monoid"):


i1 : QQ[c,x,dx,WeylAlgebra=>{c,x=>dx}]

o1 = QQ[c, x, dx]

o1 : PolynomialRing

i4 : dx*x - x*dx

     2
o4 = c

o4 : QQ[c, x, dx]
