code: function() {
      return this.read_list(
        function() {
          this.expect(this.tok.T_STRING);
          const result = this.node("constant");
          let constName = this.node("identifier");
          const name = this.text();
          this.next();
          constName = constName(name);
          if (this.expect("=")) {
            return result(constName, this.next().read_expr());
          } else {
            // fallback
            return result(constName, null);
          }
        },
        ",",
        false
      );
    }
docs: Reads a list of constants declaration
  ```ebnf
  const_list ::= T_CONST T_STRING '=' expr (',' T_STRING '=' expr)* ';'
  ```