A kit of pieces is provided comprising a buckle (1), a pair of braces and at least one elongate guide stick. The buckle comprises a first portion (2), a second portion (3), and a clamp portion (4). The first portion (2) is situated in front of the second portion (3) when the buckle (1) is in use. The clamp portion (4) is configured to clamp a first end of a belt against the second portion (3), and the first portion (2) is configured to clamp a second end of the belt against the second portion (3). Further, the first portion (2) comprises at least one groove (6).