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diff --git a/src/Calculator.g4 b/src/Calculator.g4
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+// Copyright 2021 Benjamin Barenblat
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+// The lexer and parser grammars for the EC language.
+
+grammar Calculator;
+
+options {
+  language = Cpp;
+}
+
+@parser::postinclude {
+#include <stdexcept>
+#include <string>
+#include <utility>
+
+#include "third_party/abseil/absl/strings/numbers.h"
+}
+
+program : term* ;
+
+term : number | identifier | error ;
+
+number returns [double value]
+  : s=NUMBER {
+      if (!absl::SimpleAtod($s->getText(), &$value)) {
+        throw std::runtime_error("Calculator: parser produced an invalid double");
+      }
+    }
+  ;
+
+identifier returns [std::string value]
+  : s=IDENTIFIER {
+      $value = std::move($s->getText());
+    }
+  | SUGARED_ADD { $value = "add"; }
+  | SUGARED_SUB { $value = "sub"; }
+  | SUGARED_MUL { $value = "mul"; }
+  | SUGARED_DIV { $value = "div"; }
+  ;
+
+error : ERROR ;
+
+NUMBER : [+-]? (DIGIT+ '.'? DIGIT* | '.' DIGIT+) ('e' [+-]? DIGIT+)? ;
+
+IDENTIFIER : [\p{Alpha}\p{General_Category=Other_Letter}] [\p{Alnum}\p{General_Category=Other_Letter}]* ;
+
+SUGARED_ADD : '+' ;
+SUGARED_SUB : '-' ;
+SUGARED_MUL : '*' ;
+SUGARED_DIV : '/' ;
+
+WS : [\p{White_Space}]+ -> skip ;
+
+ERROR : . ;
+
+fragment DIGIT : [0-9] ;