guile-srfi-189 final-1.659e3cd Scheme SRFI implementation of Maybe and Either

This SRFI defines two disjoint immutable container types known as Maybe and Either, both of which can contain objects collectively known as their payload. A Maybe object is either a Just object or the unique object Nothing (which has no payload); an Either object is either a Right object or a Left object. Maybe represents the concept of optional values; Either represents the concept of values which are either correct (Right) or errors (Left).