### Description This RFC proposes adding C implementation for [`@stdlib/math/base/special/sici`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/math/base/special/sici). `double stdlib_base_sici( const double x );` ### Related Issues Related issues #649 ### Questions No. ### Other No. ### Checklist - [x] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md). - [x] Searched for existing issues and pull requests. - [x] The issue name begins with `RFC:`.