public let addSpaceInsideParens = FormatRule(
        help: "Add space inside parentheses."
    )
    { formatter in
        formatter.forEach( .startOfScope( "(" ) )
        { i, _ in
            if let nextToken = formatter.token( at: i + 1 )
            {
                if !nextToken.isSpaceOrLinebreak,
                   ![ .endOfScope( ")" ) ].contains( nextToken )
                {
                    formatter.insert( .space( " " ), at: i + 1 )
                }
            }
        }
        formatter.forEach( .endOfScope( ")" ) )
        { i, _ in
            if let prevToken = formatter.token( at: i - 1 )
            {
                if !prevToken.isSpaceOrLinebreak,
                   ![ .startOfScope( "(" ) ].contains( prevToken )
                {
                    formatter.insert( .space( " " ), at: i )
                }
            }
        }
    }