added const[] accessor

src/lib/ebus/symbol.h

@@ -170,6 +170,18 @@ class SymbolString {
     return m_data[index];
   }
 
+  /**
+   * Return a reference to the symbol at the specified index.
+   * @param index the index of the symbol to return.
+   * @return the reference to the symbol at the specified index, or SYN if not available.
+   */
+  symbol_t operator[](const size_t index) const {
+    if (index >= m_data.size()) {
+      return SYN;
+    }
+    return m_data[index];
+  }
+
   /**
    * Return whether this instance is equal to the other instance.
    * @param other the other instance.