message: Describe changes required to add unions to
diff: @@ -436,7 +436,7 @@ $$
  <h2>DL-ARRAY</h2>
  <div class="section">
  <p>
  -In this section we add arrays to our basic language.
  +In this section we add arrays to our basic language, DL-1.
  Description arrays consist of an element type as well as
  an expression giving the number of elements,
  which may refer to the values of earlier fields.
  @@ -598,7 +598,18 @@ $$
  <div class="section">
  <p>
  -In this section we add unions to our basic language.
  +In this section we add unions to DL-1.
  +The parsing rules treat such unions non-deterministically;
  +implementations may produce all possible results,
  +or stop after one result has been successfully obtained.
  +
  +<p>
  +The corresponding host type is a <em>discriminated</em> union,
  +which means that expressions are tagged with $\text{inl}$ or $\text{inr}$
  +to indicate which branch was used in the result.
  +Such values can be used in a $\text{case}$ expression,
  +which evaluates one of two expressions depending on
  +the tag of the expression being tested, after it has been evaluated.
  <div class="subsection">
  <h3>Grammar</h3>
  