// f takes variable without $ sign. if it returns null, variable is kept
sS replaceDollarVars_dyn(S s, IF1<S> f) {
  if (f == null) ret s;
  ret regexpReplaceIC(s, "\\$(\\w+)", matcher -> {
    S var = matcher.group(1);
    S val = f.get(var);
    ret val == null ? matcher.group() : str(val);
  });
}

Began life as a copy of #1028680