Version 0.1.3, 2019-04-22: More caution with PROTECT. Version 0.1.2, 2018-03-26: Re-introduced two small changes to error checking accidently reverted in 0.1.1. Version 0.1.1, 2018-01-08: Small changes for compatibility with new R Core REFCNT scheme, and for more caution with PROTECT. Version 0.1.0, 2017-06-18: First version.