add
Add the receiver to the given finite integer, destroying one or the other if it's mutable and canDestroy is true. Because of the requirement that the argument be an integer rather than an arbitrary A_Number, this is usually only used for double-dispatching.
Return
The sum, possibly recycling one of the inputs if canDestroy is true.
Parameters
an
The finite integer to add to the receiver.
can
Whether a mutable receiver or argument may be destroyed and/or recycled to hold the sum.