This is a simple ad hoc parser for string templates with variables and nested functions.

Basically, the list of test strings give an idea of what it can do.

 x + y = $plus($x, $y)
 x - y = $minus($x, $y)
 10 - 7 = $minus(10, 7)
 max(9, 7) = $max(9, 7)
 max(x, y) = $max($x, $y)
 max(8, $y) = $max(8, $y)
 max(min(3,4),2)=$max($min(3,4),2)
 $max($min(3, 4), $min(10, 20))
 $sum($min(1, 2), $max(3, 4), 5); $max(3, $min($pi, 5))

All the template-related stuff begins with a dollar sign (the dollar itself is escaped as $$). Variables go as is, like $x or $y. Functions are also prefixed with a dollar as in $max(3, 14).

There's no math or whatever operations but it is possible (and it was the main goal of fstring) to use other function calls from within a function call. Thus any reasonable function enclosing sequence should be valid. For example: $max($min(1, 2), $min(3, 4)).

Note that it's only an ad hoc script placed on github for being backuped :-)