code: public static AstRoot parse(String rapids) {
      Rapids r = new Rapids(rapids);
      AstRoot res = r.parseNext();
      if (r.skipWS() != ' ')
        throw new IllegalASTException("Syntax error: illegal Rapids expression `" + rapids + "`");
      return res;
    }
docs: Parse a Rapids expression string into an Abstract Syntax Tree object.
  @param rapids expression to parse