<https://secure.gravatar.com/avatar/937b9984dcfbead52a02b22006896747?s=96&amp;d=mm&amp;r=g>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://differencess.com/covariable-vs-covariate-whats-the-difference/>