• Tr[list] finds the trace of the matrix or tensor list.
• Tr[list, f] finds a generalized trace, combining terms with f instead of Plus.
• Tr[list, f, n] goes down to level n in list.
• Tr[list] sums the diagonal elements list[[i, i, ... ]].
• Tr works for rectangular as well as square matrices and tensors.
• New in Version 4.
• Advanced Documentation.