+ added subornation logic to enough(). We do not want to mix the public
subtract() call with this method. As doing so would mean that either a)
subtraction can go below 0 (this should not happen) or b) we break this
function with exceptions. This is a work around for these scenarios.
+ calling subtraction will now throw an exception if you try and use
more mana than is available. This is better than setting it to 0.
Setting to 0 impose that you should still be allowed to perform the
action.
+ updated subtraction test to check for exception
+ subtractionCost() will not allow using mana that is not available,
same as subtract()
+ added javadoc
+ added final to method variables that should not be mutated
+ Mana can no longer be initialised with minus values
+ added null assertions
+ renamed ‘addXXX’ to ‘increaseXXX’. Add implies that you are able to
add variable amounts.
+ subtraction can no longer allow minus values.
+ removed unused state
+ refactored classes use renamed/removed mana calls
+ added comment to contains(), needs to be looked into?
+ Added validation to setting of mana. Can no longer be set to minus
values. Will log if an attempt is made
+Added comment to static mana, this is unsafe.
