'Insert a new key-value pair or modify the value in an existing
pair.
Parameters
key : hashable object
The key.
value : object comparable with existing values.
The value.
allow_increase : bool
Whether the value is allowed to increase. If False, attempts to
increase an existing value have no effect. Default value: False.
Returns
decreased : bool
True if a pair is inserted or the existing value is decreased.'

def insert(self, key, value, allow_increase=False):

    raise NotImplementedError
