From fs/reiserfs/do_balan.c:463, when CONFIG_REISERFS_CHECK is enabled:

gcc -Wp,-MD,fs/reiserfs/.do_balan.o.d -nostdinc -isystem \
  /usr/local/stow/gcc-3.4.3/lib/gcc/i686-pc-linux-gnu/3.4.3/include \
  -D__KERNEL__ -Iinclude  -Wall -Wstrict-prototypes -Wno-trigraphs \
  -fno-strict-aliasing -fno-common -ffreestanding -Os \
  -fno-omit-frame-pointer -pipe -msoft-float -mpreferred-stack-boundary=2 \
  -fno-unit-at-a-time -march=i686 -mtune=pentium2 -mregparm=3 \
  -Iinclude/asm-i386/mach-default -Wdeclaration-after-statement \
  -DKBUILD_BASENAME=do_balan -DKBUILD_MODNAME=do_balan \
  -DCONFIG_REISERFS_CHECK -E fs/reiserfs/do_balan.c > foo

You get as the output for that macro:

if( !( !( comp_short_le_keys (( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) ), ( &(( (struct item_head * )((tb->L[0])->b_data + (sizeof(struct block_head))) + ((((( __u16)(__le16)((((struct block_head *)((tb->L[0])->b_data)))->blk_nr_item))))-1) )->ih_key) )) || !item_ops[le_key_k_type (le_key_version (( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) )), ( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) ))]->is_left_mergeable (( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) ), tbS0->b_size) || !item_ops[le_key_k_type (le_key_version (( (struct reiserfs_key * )((tb->CFL[0])->b_data + (sizeof(struct block_head))) + (tb->lkey[0]) )), ( (struct reiserfs_key * )((tb->CFL[0])->b_data + (sizeof(struct block_head))) + (tb->lkey[0]) ))]->is_left_mergeable (( (struct reiserfs_key * )((tb->CFL[0])->b_data + (sizeof(struct block_head))) + (tb->lkey[0]) ), tbS0->b_size) ) ) ) reiserfs_panic( ((void *)0), "reiserfs[%i]: assertion " "!( comp_short_le_keys (( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) ), ( &(( (struct item_head * )((tb->L[0])->b_data + (sizeof(struct block_head))) + (((((__u16)(__le16)((((struct block_head *)((tb->L[0])->b_data)))->blk_nr_item))))-1) )->ih_key) )) || !item_ops[le_key_k_type (le_key_version (( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) )), ( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) ))]->is_left_mergeable (( &(( (struct item_head * )((tbS0)->b_data + (sizeof(struct block_head))) + (0) )->ih_key) ), tbS0->b_size) || !item_ops[le_key_k_type (le_key_version (( (struct reiserfs_key * )((tb->CFL[0])->b_data + (sizeof(struct block_head))) + (tb->lkey[0]) )), ( (struct reiserfs_key * )((tb->CFL[0])->b_data + (sizeof(struct block_head))) + (tb->lkey[0]) ))]->is_left_mergeable (( (struct reiserfs_key * )((tb->CFL[0])->b_data + (sizeof(struct block_head))) + (tb->lkey[0]) ), tbS0->b_size) )" " failed at " "fs/reiserfs/do_balan.c" ":%i:%s: " "PAP-12120: item must be merge-able with left neighboring item" "\n", (((current_thread_info()->preempt_count) & ((((1UL << (12))-1) << ((0 + 8) + 8)) | (((1UL << (8))-1) << (0 + 8))))) ? -1 : get_current() -> pid, 472 , (__func__) );