Mathematica 10.1 does not replace I (i.e. \[ImaginaryI], the square root of -1) inside functions with the Hold attribute. Example:
In:= Hold[I] /. I -> a Out= Hold[I]
I don't understand why this happens. I have not found any other symbols with the same behavior. If I create a new symbol with the same Attributes as I, it replaces without problem.
Why is it doing this, and is it the intended behavior?