function--treeviewer()'s problem

I haved type commands like this, and I want to know why treeviewer() can't run?

>> whos Name Size Bytes Class Attributes

BDTInstSet        1x1             15956  struct              
BDTTree           1x1              5138  struct              
BKInstSet         1x1             15946  struct              
BKTree            1x1              6036  struct              
CRRInstSet        1x1             12434  struct              
CRRTree           1x1              5058  struct              
EQPInstSet        1x1             12434  struct              
EQPTree           1x1              5058  struct              
HJMInstSet        1x1             15948  struct              
HJMTree           1x1              5838  struct              
HWInstSet         1x1             15946  struct              
HWTree            1x1              6036  struct              
ITTInstSet        1x1             12438  struct              
ITTTree           1x1              8812  struct              
ZeroInstSet       1x1             10282  struct              
ZeroRateSpec      1x1              1580  struct

>> treeviewer(HJMTree) Undefined function 'treeviewer' for input arguments of type 'struct'.