message: Describe changes required to add Booleans and conditionals to
diff: @@ -969,7 +969,19 @@ $$
  <div class="section">
  <p>
  In this section we add Booleans, integer comparisons,
  -and conditional expressions to our basic language.
  +and conditional expressions to DL-1.
  +We will use this language, DL-BOOL, a few more times
  +as the basis of languages with features that rely on Booleans.
  +For the moment, however, we just introduce the minimum functionality.
  +
  +<p>
  +The main thing to note is that we don't have a combined kind rule
  +for the new host type.
  +The reason for this is that there is no description
  +for which this type corresponds;
  +it is only used in expressions to give meaning to integer comparisons.
  +(The description $\text{int}_1(-1, 0)$ would suffice if such was needed.)
  +Consequently, we don't need a new parsing rule either.
  <div class="subsection">
  <h3>Grammar</h3>
  @@ -1095,6 +1107,10 @@ $$
  <div class="section">
  <p>
  In this section we extend DL-BOOL with propositional functions.
  +As with the evalutaion rules of DL-ARITH,
  +the occurrences of operators on the left of the $\leadsto$ symbol
  +are syntactic,
  +while those on the right are semantic.
  <div class="subsection">
  <h3>Grammar</h3>
  @@ -1124,7 +1140,7 @@ $$
  }{
  \Gamma \vdash e_1 \vee e_2 : \text{bool}
  }
  -\\[2em]
  +\qquad
  \dfrac{
  \Gamma \vdash e_1 : \text{bool} \qquad
  \Gamma \vdash e_2 : \text{bool}
  @@ -1171,7 +1187,19 @@ $$
  <div class="section">
  <p>
  -In this section we extend DL-BOOL with conditional descriptions.
  +In this section we extend DL-BOOL and DL-UNION with conditional descriptions.
  +These are similar to conditional expressions,
  +in that a choice is made based on the value of a Boolean expression.
  +In this case the choice is between two alternative descriptions,
  +rather than two alternative expressions of the same type.
  +The result of parsing is a discrimiated union,
  +with the tag indicating which branch of the construct was taken.
  +
  +<p>
  +Since we are operating at the description level
  +rather than the expression level,
  +we need kind and parsing rules instead of
  +the type and evaluation rules from before.
  <div class="subsection">
  <h3>Grammar</h3>
  