P.S.

My example was a bad one because it didn't highlight the difficulty of
parsing a formula string.

In fact in my example x()'s params were easily parsed. Just strip off
the leading "=" and outer parens, locate the commas, and you're done.
I.e.:

=x("abc",123,y(999),z(y(999)))

yields:

"abc"
"123"
"y(999)"
"z(y(999))"

The difficulty lies in **nested calls of functions with 2 or more
params**.

Using the same logic on this string:

=x("abc",123,y(999,"xyz"),z(101,y(999,"xyz")))

results in this mess:

"abc"
"123"
"y(999
"xyz)"
"z(101"
"y(999"
"xyz))"

or some such thing.

So it sher would be a great thing if Excel could do this for us.

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