Implement key parser and begin key_suffix parser.

src/parser/parser_context.rs

@@ -147,12 +147,27 @@ pub enum ContextElement<'r, 's> {
     /// The definition inside a footnote reference must have balanced
     /// brackets [] inside the definition, so this stores the amount
     /// of opening brackets subtracted by the amount of closing
-    /// brackets within the definition.
+    /// brackets within the definition must equal zero.
     ///
     /// A reference to the position in the string is also included so
     /// unbalanced brackets can be detected in the middle of an
     /// object.
     FootnoteReferenceDefinition(FootnoteReferenceDefinition<'s>),
+
+    /// Stores the current bracket depth inside a citation.
+    ///
+    /// The global prefix, global suffix, key prefix, and key suffix
+    /// inside a footnote reference must have balanced brackets []
+    /// inside the definition, so this stores the amount of opening
+    /// brackets subtracted by the amount of closing brackets within
+    /// the definition must equal zero. None of the prefixes or
+    /// suffixes can be nested inside each other so we can use a
+    /// single type for this without conflict.
+    ///
+    /// A reference to the position in the string is also included so
+    /// unbalanced brackets can be detected in the middle of an
+    /// object.
+    CitationBracket(CitationBracket<'s>),
 }
 
 pub struct ExitMatcherNode<'r> {
@@ -166,6 +181,12 @@ pub struct FootnoteReferenceDefinition<'s> {
     pub depth: usize,
 }
 
+#[derive(Debug)]
+pub struct CitationBracket<'s> {
+    pub position: &'s str,
+    pub depth: usize,
+}
+
 impl<'r> std::fmt::Debug for ExitMatcherNode<'r> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let mut formatter = f.debug_struct("ExitMatcherNode");