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