Function part(expr,0) is the same as op(expr) .
Here is what is written in the Help:
Returns the main operator of the expression expr ( op )
Returns parts of the displayed form of expr ( part(expr,0) )
inpart(expr,0) return the internal representation of the expression
gives the same result:
But ANY formal description of Function:derivative(...) is missing in the Help.
I think it is more logical to get diff in the first case, but not derivative
gives the same result as:
But again, the Help has no description of the Fucntion:derivative.