code: public boolean isPure() {
      if (soyFunction instanceof BuiltinFunction) {
        return ((BuiltinFunction) soyFunction).isPure();
      }
      return soyFunction.getClass().isAnnotationPresent(SoyPureFunction.class);
    }
docs: Whether or not this function is pure.
  
  <p>See {@link SoyPureFunction} for the definition of a pure function.