Don't have math with nested textit/texttt, because crappy MathJax doe…

…s not support it.

See mathjax/MathJax#243 and mathjax/MathJax#1758

source/lib-intro.tex

@@ -601,18 +601,18 @@
 The enumerated type \tcode{\placeholder{enumerated}} can be written:
 
 \begin{codeblock}
-enum @\placeholder{enumerated}@ { @$\tcode{\placeholder{V}}_{0}$@, @$\tcode{\placeholder{V}}_{1}$@, @$\tcode{\placeholder{V}}_{2}$@, @$\tcode{\placeholder{V}}_{3}$@, ..... };
+enum @\placeholder{enumerated}@ { @\tcode{\placeholder{V}}$_{0}$@, @\tcode{\placeholder{V}}$_{1}$@, @\tcode{\placeholder{V}}$_{2}$@, @\tcode{\placeholder{V}}$_{3}$@, ..... };
 
-inline const @$\tcode{\placeholder{enumerated C}}_{0}$@(@$\tcode{\placeholder{V}}_{0}$@);
-inline const @$\tcode{\placeholder{enumerated C}}_{1}$@(@$\tcode{\placeholder{V}}_{1}$@);
-inline const @$\tcode{\placeholder{enumerated C}}_{2}$@(@$\tcode{\placeholder{V}}_{2}$@);
-inline const @$\tcode{\placeholder{enumerated C}}_{3}$@(@$\tcode{\placeholder{V}}_{3}$@);
+inline const @\tcode{\placeholder{enumerated C}}$_{0}$@(@\tcode{\placeholder{V}}$_{0}$@);
+inline const @\tcode{\placeholder{enumerated C}}$_{1}$@(@\tcode{\placeholder{V}}$_{1}$@);
+inline const @\tcode{\placeholder{enumerated C}}$_{2}$@(@\tcode{\placeholder{V}}$_{2}$@);
+inline const @\tcode{\placeholder{enumerated C}}$_{3}$@(@\tcode{\placeholder{V}}$_{3}$@);
   .....
 \end{codeblock}
 
 \pnum
-Here, the names $\tcode{\placeholder{C}}_0$,
-$\tcode{\placeholder{C}}_1$, etc.\ represent
+Here, the names \tcode{\placeholder{C}}$_0$,
+\tcode{\placeholder{C}}$_1$, etc.\ represent
 \term{enumerated elements}
 for this particular enumerated type.
 \indextext{type!enumerated}%
@@ -637,13 +637,13 @@
 // For exposition only.
 // \tcode{int_type} is an integral type capable of representing all values of the bitmask type.
 enum @\placeholder{bitmask}@ : int_type {
-  @$\tcode{\placeholder{V}}_{0}$@ = 1 << 0, @$\tcode{\placeholder{V}}_{1}$@ = 1 << 1, @$\tcode{\placeholder{V}}_{2}$@ = 1 << 2, @$\tcode{\placeholder{V}}_{3}$@ = 1 << 3, .....
+  @\tcode{\placeholder{V}}$_{0}$@ = 1 << 0, @\tcode{\placeholder{V}}$_{1}$@ = 1 << 1, @\tcode{\placeholder{V}}$_{2}$@ = 1 << 2, @\tcode{\placeholder{V}}$_{3}$@ = 1 << 3, .....
 };
 
-inline constexpr @$\tcode{\placeholder{bitmask C}}_{0}$@(@$\tcode{\placeholder{V}}_{0}{}$@);
-inline constexpr @$\tcode{\placeholder{bitmask C}}_{1}$@(@$\tcode{\placeholder{V}}_{1}{}$@);
-inline constexpr @$\tcode{\placeholder{bitmask C}}_{2}$@(@$\tcode{\placeholder{V}}_{2}{}$@);
-inline constexpr @$\tcode{\placeholder{bitmask C}}_{3}$@(@$\tcode{\placeholder{V}}_{3}{}$@);
+inline constexpr @\tcode{\placeholder{bitmask C}}$_{0}$@(@\tcode{\placeholder{V}}$_{0}{}$@);
+inline constexpr @\tcode{\placeholder{bitmask C}}$_{1}$@(@\tcode{\placeholder{V}}$_{1}{}$@);
+inline constexpr @\tcode{\placeholder{bitmask C}}$_{2}$@(@\tcode{\placeholder{V}}$_{2}{}$@);
+inline constexpr @\tcode{\placeholder{bitmask C}}$_{3}$@(@\tcode{\placeholder{V}}$_{3}{}$@);
   .....
 
 constexpr @\placeholder{bitmask}{}@ operator&(@\placeholder{bitmask}{}@ X, @\placeholder{bitmask}{}@ Y) {
@@ -673,13 +673,13 @@
 \end{codeblock}
 
 \pnum
-Here, the names $\tcode{\placeholder{C}}_0$,
-$\tcode{\placeholder{C}}_1$, etc.\ represent
+Here, the names \tcode{\placeholder{C}}$_0$,
+\tcode{\placeholder{C}}$_1$, etc.\ represent
 \term{bitmask elements}
 for this particular bitmask type.
 \indextext{type!bitmask}%
-All such elements have distinct, nonzero values such that, for any pair $\tcode{\placeholder{C}}_i$
-and $\tcode{\placeholder{C}}_j$ where $i \neq j$, \tcode{$\placeholder{C}_i$ \& $\placeholder{C}_i$} is nonzero and
+All such elements have distinct, nonzero values such that, for any pair \tcode{\placeholder{C}}$_i$
+and \tcode{\placeholder{C}}$_j$ where $i \neq j$, \tcode{\placeholder{C}$_i$ \& \placeholder{C}$_i$} is nonzero and
 \tcode{$\placeholder{C}_i$ \& $\placeholder{C}_j$} is zero.
 \indextext{bitmask!empty}%
 Additionally, the value \tcode{0} is used to represent an \term{empty bitmask}, in which no