code: public Expression asCheap() {
      if (isCheap()) {
        return this;
      }
      return new Expression(resultType, features.plus(Feature.CHEAP)) {
        @Override
        protected void doGen(CodeBuilder adapter) {
          Expression.this.gen(adapter);
        }
      };
    }
docs: Returns an equivalent expression where {@link #isCheap()} returns {@code true}.