(* retourne un bloc libre et met à jour la liste des blocks libres *)   let alloc_block () =     if super_block.free_blocks.head = 0 then       raise (File_system_exception "File system full");     if super_block.free_blocks.nb_free == 0 then       begin         let fresh_item = super_block.free_blocks.head in         let buffer = Array.create D.block_size 0 in         D.read fresh_item buffer 0;         super_block.free_blocks.head <- buffer.(0);         super_block.free_blocks.nb_free <- D.block_size - 1;         fresh_item       end     else       begin         let buffer = Array.create D.block_size 0 in         D.read super_block.free_blocks.head buffer 0;         let fresh_item = buffer.(super_block.free_blocks.nb_free) in         super_block.free_blocks.nb_free <- super_block.free_blocks.nb_free - 1;         fresh_item       end;;