type thm: fully internal derivation, no longer exported;
incorporate former deriv.ML as internal abstract type;
added rudimentary promise interface;
tuned;
added is_named (does not require finished derivation);
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\railalias{lbrace}{\texttt{\ttlbrace}}
\railalias{rbrace}{\texttt{\ttrbrace}}
\railalias{atsign}{{\at}}
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\def\rail@termfont{\small\ttfamily\upshape}
\def\rail@tokenfont{\small\ttfamily\upshape}
\def\rail@nontfont{\small\rmfamily\upshape}
\def\rail@annofont{\small\rmfamily\itshape}
\def\rail@namefont{\small\rmfamily\itshape}
\def\rail@indexfont{\small\rmfamily\itshape}
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