clarified type Token.src: plain token list, with usual implicit value assignment;
clarified type Token.name_value, notably for head of Token.src;
clarified Attrib/Method check_src vs. parser;
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\usepackage{color}
\definecolor{linkcolor}{rgb}{0,0,0.5}
\usepackage[colorlinks=true,linkcolor=linkcolor,citecolor=linkcolor,filecolor=linkcolor,pagecolor=linkcolor,urlcolor=linkcolor]{hyperref}