def incr(self, key, delta=<NUM_LIT:1>):
return self._incrdecr("<STR_LIT>", key, delta)<EOL>
Sends a command to the server to atomically increment the value
for C{key} by C{delta}, or by 1 if C{delta} is unspecified.
Returns None if C{key} doesn't exist on server, otherwise it
returns the new value after incrementing.

Note that the value for C{key} must already exist in the memcache,
and it must be the string representation of an integer.

>>> mc.set("counter", "20")  # returns 1, indicating success
1
>>> mc.incr("counter")
21
>>> mc.incr("counter")
22

Overflow on server is not checked.  Be aware of values approaching
2**32.  See L{decr}.

@param delta: Integer amount to increment by (should be zero or greater).
@return: New value after incrementing.
@rtype: int