code: function() {
      switch (this.token) {
        case this.tok.T_FUNCTION:
          return this.read_function(false, false);
        // optional flags
        case this.tok.T_ABSTRACT:
        case this.tok.T_FINAL:
        case this.tok.T_CLASS:
          return this.read_class();
        case this.tok.T_INTERFACE:
          return this.read_interface();
        case this.tok.T_TRAIT:
          return this.read_trait();
        case this.tok.T_USE:
          return this.read_use_statement();
        case this.tok.T_CONST: {
          const result = this.node("constantstatement");
          const items = this.next().read_const_list();
          this.expectEndOfStatement();
          return result(null, items);
        }
        case this.tok.T_NAMESPACE:
          return this.read_namespace();
        case this.tok.T_HALT_COMPILER: {
          const result = this.node("halt");
          if (this.next().expect("(")) this.next();
          if (this.expect(")")) this.next();
          this.expect(";");
          this.lexer.done = true;
          return result(this.lexer._input.substring(this.lexer.offset));
        }
        default:
          return this.read_statement();
      }
    }
docs: reading a top statement
  ```ebnf
  top_statement ::=
  namespace | function | class
  | interface | trait
  | use_statements | const_list
  | statement
  ```