Add R^dxd and R^d expressions in pugs' language (with d=1,2 or 3)
These expressions are defined as follow
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for R^d: [a,b,...], where the d expressions (a,b,...) convert naturally to R
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for R^dxd: [[a,b,...],[c,d,...],...], where the dxd expressions (a,b,c,d,...) convert naturally to R