Add R^dxd and R^d expressions in pugs' language (with d=1,2 or 3)

These expressions are defined as follow

- for R^d: [a,b,...], where the d expressions (a,b,...) convert
naturally to R

- for R^dxd: [[a,b,...],[c,d,...],...], where the dxd
expressions (a,b,c,d,...) convert naturally to R