Is it possible to change the output of build-in functions?
I would like to change the output for IdentityMatrix[n] to "I", so I tried
Format[IdentityMatrix[n]] := "I"
For comparison I also tried
Format[Sin[x_]] := "Sine(" <> ToString[x] <> ")"
Both result into an error: "Tag IdentityMatrix in IdentityMatrix[n] is Protected."
I guess these functions are protected because they are build-in functions. Is there a way to change the output format of a build-in function?