how to do subs for symmetric indexed objects?

[Yong Xiao]

Hello,
I am tring to substitute Y_{imn}t_{nj} by Y_{jmn}t_{ni} where Y is a 
symmetric indexed object. My code for this is like
indexed(Y,sy_symm(),i_,m_,n_)*indexed(t,n_,j_)
==indexed(Y,sy_symm(),j_,n_,m_)*indexed(t,n_,i_)
where i_,j_,m_ and n_ are wildcards. But the code doesn't work. Attached 
  is the code.
How should I do? Thanks
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