Solve the programming task below in a Python markdown code block.
# Exclusive "or" (xor) Logical Operator

## Overview

In some scripting languages like PHP, there exists a logical operator (e.g. ```&&```, ```||```, ```and```, ```or```, etc.) called the "Exclusive Or" (hence the name of this Kata).  The exclusive or evaluates two booleans.  It then returns true if **exactly one of the two expressions are true**, false otherwise.  For example:

## Task

Since we cannot define keywords in Javascript (well, at least I don't know how to do it), your task is to define a function ```xor(a, b)``` where a and b are the two expressions to be evaluated.  Your ```xor``` function should have the behaviour described above, returning true if **exactly one of the two expressions evaluate to true**, false otherwise.
Also feel free to reuse/extend the following starter code:
```python
def xor(a,b):
```