I want an indexed identifier f_i for various i, so I make it a parametrized macro. The index is self-delimiting---I don't need parentheses in f_i+j---so I define it to strip parentheses from the index:

%format f (x) = f "_{" x "}"

But I also want to format it as a single token, even when it is passed as an argument to something else---I also don't want parentheses in h f_i+j---so I also strip off parentheses from the outside:

%format (f (x)) = f "_{" x "}"

This works for the original use case h f_i+j:

h (f (i+j))

and is as described in Section 7.5 of Guide2.pdf.

But it does not work as I expect when I apply f_i+j to an argument x then apply h to the result:

h (f (i+j) x)

I get h f_i+j x, and I think some non-redundant parentheses are being stripped off, namely those around the application of f_i+j to x.

To get round this, I have to supply some extra (I think redundant) parentheses---either round the indexed function, to satisfy its hunger:

h ((f (i+j)) x)

or an identical second pair around the whole argument to h:

h ((f (i+j) x))

Sample input (as TXT because Github blocks LHS) and output attached.

brackets.pdf
brackets.txt