[IR PASS]Questions about FmutateStmt

Questions
#1

Recently I am studing IR PASS about VTA, and I am so confused about vtable_expr()``vtable_stmt().
I follow the PASS of inject_dma_intrin ,and I found that it will call CopyIntrinInjector.Mutate in inject_copy_intrin.cc, and the function Mutate is defined as following:

  virtual Stmt Mutate(Stmt stmt) {
    static const FMutateStmt& f = vtable_stmt();
    return f(stmt, stmt, this);
  }

I don’t understand what is the vtable_stmt() meaning, can you help me ?
Thanks!