// a rule that has a string as LHS and multiple strings as RHS,
// plus an optional list of variables (usually a ciSet)
srecord Rule_StringToStrings(S in, LS out, Set<S> vars) {
  *(S *in) {}
  *(S *in, LS *out) {}
  
  toString {
    ret "Rule_StringToStrings: " + in + " :=\n"
      + (vars == null ? "" : indentx("vars " + curly(joinWithComma(vars))) + "\n")
      + indentx(pnlToString("out ", out));
  }
}