Hi,

Right now, TeX-fold-expand-spec does not abort its expansion of the
current spec if *any* of the expansions succeeded. This can lead to
some unexpected behaviour. For example, given

    (setopt TeX-fold-math-spec-list
            '(("[1] of {1}||{1}" ("cite"))))

expanding `\cite{this}' would result in the overlay displaying
`… of this' instead of just `this', because one of the arguments (the
mandatory one) succeeded. This is probably not what the user intended.
Instead, in order for the spec to be chosen, *all* arguments should
succeed.

The attached patch does this.

One thing that seems related is that the function claims that it also
matches arguments of the form `(<number>)', but the regular expression
that's used

    "\\([[{<]\\)\\([1-9]\\)\\([]}>]\\)"

does not contain any matches for simple parentheses. I don't know
whether I'm missing something here, so I didn't change this.

Best,
  Tony