Add support for redefining operators

corpus/subprograms.txt

@@ -660,3 +660,49 @@ function F (A : My_Array) return Boolean
                     (factor
                       (primary
                         (numeric_literal)))))))))))))
+
+================================================================================
+Operators
+================================================================================
+
+overriding function "<" (Left, Right : My_Int) return Boolean is
+begin
+   return False;
+end "<";
+
+--------------------------------------------------------------------------------
+
+(compilation
+  (compilation_unit
+    (proper_body
+      (subprogram_body
+        (overriding_indicator)
+        (subprogram_specification
+          (function_specification
+            (string_literal)
+            (parameter_and_result_profile
+              (formal_part
+                (parameter_specification_list
+                  (parameter_specification
+                    (defining_identifier_list
+                      (identifier)
+                      (identifier))
+                    (name
+                      (identifier)))))
+              (result_profile
+                (name
+                  (identifier))))))
+        (handled_sequence_of_statements
+          (sequence_of_statements
+            (statement
+              (simple_statement
+                (simple_return_statement
+                  (expression
+                    (relation
+                      (simple_expression
+                        (term
+                          (factor
+                            (primary
+                              (name
+                                (identifier)))))))))))))
+        (string_literal)))))

grammar.js

@@ -382,7 +382,10 @@ module.exports = grammar({
          reservedWord('begin'),
          $.handled_sequence_of_statements,
          reservedWord('end'),
-         optional($.name),
+         optional(choice(
+            $.name,
+            $.string_literal,  //  for operators
+         )),
          ';'
       ),
       package_body: $ => seq(
@@ -1257,7 +1260,10 @@ module.exports = grammar({
       ),
       function_specification: $ => seq(
          reservedWord('function'),
-         $.name,
+         choice(
+            $.name,
+            $.string_literal,  // for operators
+         ),
          $.parameter_and_result_profile,
       ),
       generic_declaration: $ => choice(