Question: typesetting

hi i have one question in maple is many function which can have two 2D output like sum: Sum(f,i=3..n); in 2d Sum(f,i=3..n); sum(f,i=3..n); in 2d (n-2)f; for example i wanna my own function which will has two forms like a sum but in unevalued form will be instead of sigma for example Q or another symbol and above and belove that symbol will be some values, and in evaluated form there will be expression or number; another example is Diff or BesselJ funcions which have both two forms (symbolic/algebraic) my question is how to make this? Q(4,2); 2 Q 4 q(4,2); 8 but Q have to has a size like sigma in sum and 2,4 size as indexes... i thing there is possibility to define a typesetting rule but I don't know how...
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