code: func matchSingleLiteral(expr syntax.Expr) interface{} {
  	if literal, ok := expr.(*syntax.Literal); ok {
  		return literal.Value
  	}
  	return nil
  }
docs: // matchSingleLiteral matches an <Expr> to <Literal>, returning literal's value.
  //
  // The returned value is string | int64 | *big.Int.